feat: add OCR text extraction and Postgres full-text search

- Create ocr_text table for storing per-region OCR results
- Add tsvector search_vector column to photos with GIN index and
  auto-update trigger on filename/user_title/user_notes
- Implement ocr_photo Celery task using rapidocr-onnxruntime
- Add FTS leg to hybrid search: queries photos.search_vector and
  ocr_text via UNION, fused with semantic results via RRF (k=60)

Migration 0004 backfills search_vector for existing rows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/search.py

@@ -62,9 +62,30 @@ async def hybrid_search(
         except Exception as e:
             logger.warning("Semantic search failed (models may not be loaded): %s", e)
 
-    # ── FTS search (placeholder for PR5) ──────────────────────────────
-    # Will be: tsvector @@ plainto_tsquery(q), ranked by ts_rank.
-    # For now, skip.
+    # ── FTS search (photos.search_vector + ocr_text) ────────────────
+    if q:
+        try:
+            fts_stmt = text("""
+                SELECT id, ts_rank(search_vector, plainto_tsquery('english', :q)) AS rank
+                FROM photos
+                WHERE search_vector @@ plainto_tsquery('english', :q)
+                UNION
+                SELECT o.photo_id AS id,
+                       MAX(o.confidence) AS rank
+                FROM ocr_text o
+                WHERE to_tsvector('english', o.text) @@ plainto_tsquery('english', :q)
+                GROUP BY o.photo_id
+                ORDER BY rank DESC
+                LIMIT 200
+            """)
+            fts_rows = (await db.execute(fts_stmt, {"q": q})).fetchall()
+            for rank, (photo_id, score) in enumerate(fts_rows):
+                if photo_id not in results:
+                    results[photo_id] = {"semantic_rank": None, "fts_rank": rank}
+                else:
+                    results[photo_id]["fts_rank"] = rank
+        except Exception as e:
+            logger.warning("FTS search failed: %s", e)
 
     # ── RRF fusion ────────────────────────────────────────────────────
     k = 60