feat(halacha): crash-safe incremental extraction + resume (A + resume)

Halacha extraction held ALL chunk results in memory and stored once at the very
end — a crash/interrupt mid-run (e.g. the 2026-05-31 freeze) lost everything and
re-paid the full LLM cost on retry.

Now each chunk's halachot are stored AND the chunk is checkpointed
(precedent_chunks.halacha_extracted_at) the moment it finishes:

- V25 schema: precedent_chunks.halacha_extracted_at (per-chunk checkpoint).
- db.store_halachot_for_chunk: atomic per-chunk insert (halacha_index continues
  from MAX, caller serializes via an in-process store-lock) + checkpoint mark.
- db.reset_halacha_extraction (force) / mark_all_chunks_extracted (legacy backfill).
- _extract_impl rewritten: resume by default (skip checkpointed chunks; failed
  chunks stay pending and are retried; status stays 'processing' until all done);
  force=True wipes + redoes all. reextract_halachot passes force=True; the queue
  drain (process_pending) resumes by default.
- Legacy guard: a pre-V25 precedent (halachot exist, no checkpoints) is
  backfilled and treated as complete — never re-extracted (would duplicate).

Verified on 9002-24 (55 halachot, legacy): resume → legacy-backfill, NO
duplication (stays 55), all chunks checkpointed. Index continuation: store at
55,56 after max 54, no collision. Tracks #72.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/precedent_library.py

@@ -156,7 +156,10 @@ async def reextract_halachot(
     # bad data. See note in db.request_metadata_extraction.
 
     await progress("extracting_halachot", 50, "מחלץ הלכות מחדש")
-    result = await halacha_extractor.extract(case_law_id)
+    # Explicit re-extraction = clean slate (force): wipe prior halachot +
+    # per-chunk checkpoints and redo all. (Queue draining / resume uses the
+    # default force=False so an interrupted run continues where it stopped.)
+    result = await halacha_extractor.extract(case_law_id, force=True)
     # Clear the queue timestamp on completion so the UI badge / worker queue
     # don't keep showing this row. The queue worker (process_pending_extractions)
     # already does this; mirror it here so per-record extraction drains too.