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/src/legal_mcp/services/digest_library.py

@@ -144,7 +144,7 @@ async def create_pending_digest(
         raise ValueError(f"file not found: {file_path}")
 
     await progress("staging", 10, "מעתיק קובץ")
-    staged = ingest._stage_file(src, DIGEST_LIBRARY_DIR, "incoming")
+    staged = await ingest._stage_file(src, DIGEST_LIBRARY_DIR, "incoming")
     rel_path = str(staged.relative_to(config.DATA_DIR)) \
         if str(staged).startswith(str(config.DATA_DIR)) else str(staged)