fix(retrieval): rewrite chunk-page retrofit to skip OCR

The first-pass retrofit re-extracted via extractor.extract_text, which
re-runs Google Vision OCR on scanned pages. OCR is non-deterministic,
so the new text didn't match the chunk content stored in the DB
(produced by the original OCR run) — only ~7% of chunks were located.

New approach (no OCR cost):

1. Use the stored documents.extracted_text from the DB — the exact
   text the chunks were produced from, so chunk lookups match.
2. Anchor page boundaries via PyMuPDF direct text reads (free, no
   OCR). Pages with usable direct text are anchored by snippet match;
   OCR-only pages are linearly interpolated between anchors.
3. Search each chunk in extracted_text using a whitespace-tolerant
   helper — needed because the chunker joins paragraphs with single
   '\\n' while extracted_text uses '\\n\\n' as page separators.

Verified on 8174-24 (5 docs, 307 chunks) + 8137-24 (9 docs, 512
chunks): 100% chunks tagged, 13s total, $0 cost.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/backfill_chunk_pages.py

@@ -1,36 +1,42 @@
-"""Backfill page_number on existing document_chunks.
+"""Backfill page_number on existing document_chunks (no re-OCR).
 
 Why this exists: the legacy chunker did not track which page each chunk
 came from. After the page-tracking fix, new uploads carry page_number
 correctly, but existing chunks have ``page_number=NULL`` in the DB.
-That blocks the multimodal hybrid retriever's text+image boost (it
+That blocks the multimodal hybrid retriever's text+image boost (which
 joins (chunk, image) on (document_id, page_number)).
 
-What it does (per case):
-    1. List every document in the case
-    2. For each document with NULL page_number chunks:
-       a. Re-extract via extractor.extract_text (re-runs OCR if needed —
-          ~$0.0015/page on Google Vision; idempotent on the DB side)
-       b. Compute page_offsets from the re-extracted text
-       c. For every chunk row (sorted by chunk_index), search its
-          content in the re-extracted text → look up page → UPDATE
-    3. Skip documents whose chunks already have non-null page_number
+What it does (per case, per document):
+
+    1. Load stored ``documents.extracted_text`` from the DB. This is
+       the exact text that was used to produce the existing chunks —
+       so chunk content lookups against it match verbatim.
+    2. Open the PDF with PyMuPDF and call ``page.get_text()`` on each
+       page (cheap, no OCR). For pages with usable direct text we get
+       a clean snippet; for fully-scanned pages we get little/nothing.
+    3. Anchor: for each page with a usable snippet, search the snippet
+       in ``extracted_text`` to recover that page's start offset.
+    4. Interpolate: for OCR-only pages with no anchor, position is
+       linearly interpolated between the nearest anchored neighbors
+       (or uniformly when no anchors exist at all).
+    5. For every chunk row (sorted by chunk_index), find the chunk's
+       content in ``extracted_text`` (verbatim match), look up the
+       page from the offsets, and ``UPDATE document_chunks SET
+       page_number = ?``.
 
 Idempotent: a second run with no --force is a no-op.
 
-Designed to run from inside the FastAPI/MCP container (where /data
-is mounted and Google Vision creds are present). Locally it requires
-GOOGLE_CLOUD_VISION_API_KEY in ~/.env.
+Cost: zero. Runs in seconds even for the 89-page appraisal report.
 
 Usage:
-    docker exec -it <legal-ai-container> python /tmp/backfill_chunk_pages.py 8174-24 8137-24
+    docker cp scripts/backfill_chunk_pages.py <c>:/tmp/
+    docker exec <c> python /tmp/backfill_chunk_pages.py 8174-24 8137-24
 """
 from __future__ import annotations
 
 import argparse
 import asyncio
 import logging
-import os
 import sys
 import time
 from pathlib import Path
@@ -45,7 +51,8 @@ def _setup_paths():
 
 
 _setup_paths()
-from legal_mcp.services import db, extractor  # noqa: E402
+import fitz  # PyMuPDF  # noqa: E402
+from legal_mcp.services import db  # noqa: E402
 
 logging.basicConfig(
     level=logging.INFO,
@@ -54,6 +61,14 @@ logging.basicConfig(
 logger = logging.getLogger("backfill_chunk_pages")
 
 
+# Snippet length for page anchoring. Long enough to be unique, short
+# enough to survive minor whitespace variation between PyMuPDF direct
+# extraction and the stored OCR text.
+ANCHOR_SNIPPET_LEN = 80
+# Minimum direct-text length on a page to attempt anchoring at all.
+MIN_DIRECT_LEN = 60
+
+
 def _resolve_local_path(db_path: str) -> Path:
     p = Path(db_path)
     if p.is_file():
@@ -65,6 +80,123 @@ def _resolve_local_path(db_path: str) -> Path:
     return p
 
 
+def _norm_whitespace(s: str) -> str:
+    """Collapse runs of whitespace; helps cross-source matching where
+    PyMuPDF direct extraction may differ from the stored OCR text in
+    line-break placement."""
+    return " ".join(s.split())
+
+
+def _find_anchored_snippet(
+    extracted_text: str, snippet: str, search_start: int = 0,
+) -> int:
+    """Search for ``snippet`` in ``extracted_text``, tolerant to
+    whitespace differences. Returns the offset in the original
+    extracted_text, or -1."""
+    # Direct match first — fastest path
+    idx = extracted_text.find(snippet, search_start)
+    if idx >= 0:
+        return idx
+    # Whitespace-normalized fallback
+    norm_text = _norm_whitespace(extracted_text)
+    norm_snip = _norm_whitespace(snippet)
+    if not norm_snip:
+        return -1
+    norm_idx = norm_text.find(norm_snip)
+    if norm_idx < 0:
+        return -1
+    # Map norm offset back to original — count chars until we've passed
+    # `norm_idx` non-collapsed characters in the original.
+    orig_pos = 0
+    norm_pos = 0
+    in_ws = False
+    for ch in extracted_text:
+        if norm_pos == norm_idx:
+            return orig_pos
+        if ch.isspace():
+            if not in_ws:
+                norm_pos += 1
+                in_ws = True
+        else:
+            in_ws = False
+            norm_pos += 1
+        orig_pos += 1
+    return -1
+
+
+def _compute_page_offsets(pdf_path: Path, extracted_text: str) -> list[int]:
+    """Return ``page_offsets`` (start char offset of each page in
+    ``extracted_text``), using direct PyMuPDF reads for anchoring and
+    linear interpolation for OCR-only pages."""
+    doc = fitz.open(str(pdf_path))
+    n_pages = len(doc)
+    anchors: list[int | None] = [None] * n_pages
+
+    last_pos = 0
+    for i, page in enumerate(doc):
+        direct = page.get_text().strip()
+        if len(direct) < MIN_DIRECT_LEN:
+            continue
+        # Take the first ANCHOR_SNIPPET_LEN chars after stripping
+        snippet = direct[:ANCHOR_SNIPPET_LEN]
+        pos = _find_anchored_snippet(extracted_text, snippet, last_pos)
+        if pos < 0:
+            # try a global search before giving up
+            pos = _find_anchored_snippet(extracted_text, snippet, 0)
+        if pos >= 0:
+            anchors[i] = pos
+            last_pos = pos
+    doc.close()
+
+    # Force first page to start at 0 if not already anchored
+    if anchors[0] is None:
+        anchors[0] = 0
+
+    # Fill gaps via linear interpolation between the nearest anchors;
+    # extrapolate beyond the last anchor by the average page length.
+    page_offsets: list[int] = [0] * n_pages
+    for i in range(n_pages):
+        if anchors[i] is not None:
+            page_offsets[i] = anchors[i]
+            continue
+        # Find prev anchored
+        prev_i = i - 1
+        while prev_i >= 0 and anchors[prev_i] is None:
+            prev_i -= 1
+        # Find next anchored
+        next_i = i + 1
+        while next_i < n_pages and anchors[next_i] is None:
+            next_i += 1
+        prev_pos = anchors[prev_i] if prev_i >= 0 else 0
+        if next_i < n_pages:
+            next_pos = anchors[next_i]
+            ratio = (i - prev_i) / (next_i - prev_i)
+            page_offsets[i] = int(prev_pos + ratio * (next_pos - prev_pos))
+        else:
+            # Extrapolate: assume uniform distribution beyond last anchor
+            # using page-density inferred from prior anchors (or fall
+            # back to total_text/n_pages).
+            avg = len(extracted_text) / max(1, n_pages)
+            page_offsets[i] = int(prev_pos + avg * (i - prev_i))
+    # Monotone-clip just in case interpolation ever goes backwards
+    for i in range(1, n_pages):
+        if page_offsets[i] < page_offsets[i - 1]:
+            page_offsets[i] = page_offsets[i - 1]
+    return page_offsets
+
+
+def _page_at_offset(offset: int, page_offsets: list[int]) -> int:
+    if not page_offsets:
+        return 1
+    page = 1
+    for i, start in enumerate(page_offsets):
+        if start <= offset:
+            page = i + 1
+        else:
+            break
+    return page
+
+
 async def _backfill_document(
     document_id: UUID,
     title: str,
@@ -73,7 +205,6 @@ async def _backfill_document(
 ) -> dict:
     pool = await db.get_pool()
 
-    # Fetch chunks for this document
     chunks = await pool.fetch(
         "SELECT id, chunk_index, content, page_number FROM document_chunks "
         "WHERE document_id = $1 ORDER BY chunk_index",
@@ -94,15 +225,21 @@ async def _backfill_document(
     if pdf_path.suffix.lower() != ".pdf":
         return {"status": "not_pdf"}
 
-    logger.info("  re-extracting %s (%d chunks, %d need page)",
-                title, len(chunks), n_null)
-    t0 = time.time()
-    text, page_count, page_offsets = await extractor.extract_text(str(pdf_path))
-    elapsed = time.time() - t0
-    if not page_offsets:
-        return {"status": "no_offsets"}
+    doc_row = await pool.fetchrow(
+        "SELECT extracted_text FROM documents WHERE id = $1", document_id,
+    )
+    extracted_text = doc_row["extracted_text"] if doc_row else None
+    if not extracted_text:
+        return {"status": "no_extracted_text"}
 
-    # Walk chunks, find each in the re-extracted text, assign page
+    t0 = time.time()
+    page_offsets = _compute_page_offsets(pdf_path, extracted_text)
+    n_anchored = sum(1 for i in range(len(page_offsets)) if i == 0 or page_offsets[i] > page_offsets[i - 1])
+
+    # The chunker joins paragraphs with single `\n` while extracted_text
+    # has `\n\n` between pages, so verbatim search misses cross-page
+    # chunks. Use the whitespace-tolerant helper that returns an offset
+    # in the *original* text.
     pos = 0
     updated = 0
     not_found = 0
@@ -110,29 +247,34 @@ async def _backfill_document(
         content = c["content"]
         if not content:
             continue
-        idx = text.find(content, pos)
+        # Use a unique slice from the chunk to anchor in extracted_text
+        # — anchoring on the chunk's first ~120 chars is enough to
+        # disambiguate across the document.
+        snippet = content[: min(len(content), 120)]
+        idx = _find_anchored_snippet(extracted_text, snippet, pos)
         if idx < 0:
-            idx = text.find(content)  # global fallback
+            idx = _find_anchored_snippet(extracted_text, snippet, 0)
         if idx < 0:
             not_found += 1
             continue
-        page = extractor.page_at_offset(idx, page_offsets)
+        page = _page_at_offset(idx, page_offsets)
         await pool.execute(
             "UPDATE document_chunks SET page_number = $1 WHERE id = $2",
             page, c["id"],
         )
         updated += 1
-        # advance roughly past midpoint — chunks have overlap
         pos = idx + max(1, len(content) // 2)
 
+    elapsed = time.time() - t0
     logger.info(
-        "  done in %.1fs: extracted %d pages, updated %d/%d chunks, "
-        "%d not found", elapsed, page_count, updated, len(chunks), not_found,
+        "  %s — %d pages, %d anchors, updated %d/%d chunks (%d not found) in %.2fs",
+        title, len(page_offsets), n_anchored, updated, len(chunks), not_found, elapsed,
     )
     return {
         "status": "ok",
-        "elapsed_sec": round(elapsed, 1),
-        "pages": page_count,
+        "elapsed_sec": round(elapsed, 2),
+        "pages": len(page_offsets),
+        "anchors": n_anchored,
         "chunks_total": len(chunks),
         "chunks_updated": updated,
         "chunks_not_found": not_found,
@@ -168,7 +310,7 @@ async def backfill_cases(case_numbers: list[str], force: bool) -> dict:
             "skipped": sum(1 for r in per_doc if r["status"] == "skipped"),
             "missing": sum(1 for r in per_doc if r["status"] == "missing"),
             "no_chunks": sum(1 for r in per_doc if r["status"] == "no_chunks"),
-            "no_offsets": sum(1 for r in per_doc if r["status"] == "no_offsets"),
+            "no_extracted_text": sum(1 for r in per_doc if r["status"] == "no_extracted_text"),
             "chunks_updated": sum(r.get("chunks_updated", 0) for r in per_doc),
             "documents": per_doc,
         }
@@ -176,11 +318,11 @@ async def backfill_cases(case_numbers: list[str], force: bool) -> dict:
 
 
 def main():
-    parser = argparse.ArgumentParser(description="Backfill page_number on existing chunks")
+    parser = argparse.ArgumentParser(description="Backfill page_number on existing chunks (no OCR)")
     parser.add_argument("cases", nargs="+", help="Case numbers (e.g. 8174-24 8137-24)")
     parser.add_argument(
         "--force", action="store_true",
-        help="Re-extract even if all chunks already have page_number (default: skip)",
+        help="Re-process even if all chunks already have page_number (default: skip)",
     )
     args = parser.parse_args()
 
@@ -195,8 +337,8 @@ def main():
             continue
         print(
             f"  {cn}: {s['documents_total']} docs — "
-            f"ok {s['ok']}, skipped {s['skipped']}, missing {s['missing']}, "
-            f"chunks_updated {s['chunks_updated']}"
+            f"ok {s['ok']}, skipped {s['skipped']}, "
+            f"missing {s['missing']}, chunks_updated {s['chunks_updated']}"
         )