feat(retrieval): add voyage-multimodal-3 page-image embeddings (feature flag)

Stage C: per-page image embeddings via voyage-multimodal-3 + hybrid
text+image search. Off by default; enable with MULTIMODAL_ENABLED=true.

- Schema V9: document_image_embeddings + precedent_image_embeddings
  (vector(1024), page_number, image_thumbnail_path)
- extractor.render_pages_for_multimodal renders PDF pages at
  MULTIMODAL_DPI (144) for embedding + JPEG thumbnails at
  MULTIMODAL_THUMB_DPI (96) for UI preview, in one pass
- embeddings.embed_images calls voyage-multimodal-3 in 50-page batches
- services/hybrid_search.py orchestrator: rerank applied to text side
  first (rerank-2 is text-only); image side cosine; weighted merge
  with text_weight 0.65 (env-tunable); image-only pages surface as
  match_type='image' so dense scanned content still appears
- processor.process_document and precedent_library.ingest_precedent
  gated by flag — non-fatal on multimodal failure
- scripts/multimodal_backfill.py — idempotent per-case CLI to embed
  existing documents without re-extracting text

Validated locally on a 5-page response brief: render 0.31s, embed 8.32s,
hybrid merge surfaces image rows correctly. Production rollout starts
with flag=false (no behavior change), then per-case A/B.

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

scripts/SCRIPTS.md

@@ -22,6 +22,7 @@
 | `voyage_multimodal_poc.py` | python | POC #3 — voyage-multimodal-3 על דוח שמאי (89 עמודים). הכרעה: שיפור משמעותי לטבלאות + 22 עמודי image-only שhttp text-OCR מאבד | בנצ'מרק חד-פעמי, מוכן לשלב C |
 | `voyage_rerank_judge_poc.py` | python | POC #4 — voyage-3 vs rerank-2 vs context-3 על אהרון ברק, 18 שאילתות, claude-haiku-4-5 כ-judge. הכרעה: rerank-2 ניצח עם +9% mean@3 | בנצ'מרק חד-פעמי |
 | `voyage_rerank_corpus_poc.py` | python | POC #5 — voyage-3 vs rerank-2 על קורפוס מלא (785 docs). הכרעה: +4.5% mean@3 כללי, +11.6% על P queries (practical) | בנצ'מרק חד-פעמי, אישר את שלב B |
+| `multimodal_backfill.py` | python | Backfill voyage-multimodal-3 page embeddings על מסמכי תיקים קיימים. idempotent (skips by default), forces `MULTIMODAL_ENABLED=true` ל-run, רץ מהקונטיינר. שלב C — ראה `docs/voyage-upgrades-plan.md` | ידני per-case (`python multimodal_backfill.py 8174-24 8137-24`) |
 
 ## תיקיית `.archive/` — סקריפטים שהושלמו