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/models/__init__.py

@@ -6,6 +6,7 @@ from app.models.folders import Folder, SourceRoot
 from app.models.tags import Tag, PhotoTag
 from app.models.heaps import Heap, HeapPhoto
 from app.models.embeddings import Embedding
+from app.models.ocr_text import OCRText
 
 __all__ = [
     'Photo',
@@ -15,5 +16,6 @@ __all__ = [
     'PhotoTag',
     'Heap',
     'HeapPhoto',
-    'Embedding'
+    'Embedding',
+    'OCRText',
 ]