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/ocr_text.py

@@ -0,0 +1,20 @@
+"""
+OCR text model — stores text regions extracted from photos via rapidocr.
+"""
+from sqlalchemy import Column, String, Float, ForeignKey, Text, DateTime, func
+from sqlalchemy.dialects.postgresql import JSONB
+import uuid
+
+from app.database import Base
+
+
+class OCRText(Base):
+    __tablename__ = 'ocr_text'
+
+    id = Column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
+    photo_id = Column(String, ForeignKey('photos.id', ondelete='CASCADE'), nullable=False, index=True)
+    text = Column(Text, nullable=False)
+    language = Column(String(8), default='')
+    confidence = Column(Float)
+    bbox = Column(JSONB)  # [x1, y1, x2, y2] normalized 0-1
+    created_at = Column(DateTime(timezone=True), server_default=func.now())