feat(storage): X14 Phase 2a — route source-document writes through storage.py

Rewire the source-document staging writes onto the unified storage layer
(INV-STG1), replacing direct shutil.copy2 calls:
- tools/documents.py: case originals + training-corpus uploads
- services/ingest.py: _stage_file (now async) — covers precedent-library,
  internal-decisions, and digests (the canonical intake helper)
- services/digest_library.py: awaits the now-async _stage_file

Each write goes through storage.put_file(..., bucket=DOCUMENTS) with the
DATA_DIR-relative key; the Hebrew original filename rides as object metadata
(INV-STG2), content-type is guessed from the extension. DB path columns are
unchanged (still the absolute dest) — object_key backfill is Phase 3.

Under the default STORAGE_BACKEND=filesystem the bytes land at the exact
legacy on-disk location (put_file → shutil.copy2 to DATA_DIR/key), so this
is zero behaviour change in prod. shutil import dropped where now unused.

tests: +2 staging regression tests (file lands under DATA_DIR at the legacy
path); 20 storage + 22 ingest tests green; 242 collected with no import
breakage.

Derived/export write sites (thumbnails, extracted text, DOCX exports) are
Phase 2b. Keeps G2; advances INV-STG1. Spec: docs/spec/X14-storage-minio.md.

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

mcp-server/tests/test_storage_staging.py

@@ -0,0 +1,48 @@
+"""Regression tests for the write call-sites rewired onto storage.py (X14
+Phase 2). They assert the rewired staging lands bytes at the exact legacy
+on-disk location under the default filesystem backend — i.e. zero behaviour
+change.
+"""
+import asyncio
+from pathlib import Path
+
+import pytest
+
+from legal_mcp import config
+from legal_mcp.services import ingest, storage
+
+
+@pytest.fixture(autouse=True)
+def _tmp_datadir(tmp_path, monkeypatch):
+    monkeypatch.setattr(config, "DATA_DIR", tmp_path)
+    monkeypatch.setattr(config, "STORAGE_BACKEND", "filesystem")
+    storage.reset_storage_cache()
+    yield tmp_path
+    storage.reset_storage_cache()
+
+
+def run(coro):
+    return asyncio.run(coro)
+
+
+def test_stage_file_lands_under_datadir(_tmp_datadir):
+    src = _tmp_datadir / "src" / "כתב ערר.pdf"
+    src.parent.mkdir(parents=True)
+    src.write_bytes(b"%PDF-1.4 ...")
+    root = _tmp_datadir / "precedent-library"
+    dest = run(ingest._stage_file(src, root, "court_ruling"))
+    # dest is under the staging subdir, prefixed with a uuid, original suffix kept
+    assert dest.parent == root / "court_ruling"
+    assert dest.exists()
+    assert dest.read_bytes() == b"%PDF-1.4 ..."
+    assert dest.suffix == ".pdf"
+    # and the key is DATA_DIR-relative (what the DB column will store)
+    assert dest.relative_to(_tmp_datadir).as_posix().startswith("precedent-library/court_ruling/")
+
+
+def test_stage_file_default_subdir(_tmp_datadir):
+    src = _tmp_datadir / "x.docx"
+    src.write_bytes(b"doc")
+    dest = run(ingest._stage_file(src, _tmp_datadir / "digests", ""))
+    assert dest.parent == _tmp_datadir / "digests" / "other"
+    assert dest.exists()