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/alembic/versions/0004_ocr_text_and_fts.py

@@ -0,0 +1,82 @@
+"""ocr_text table and Postgres FTS
+
+Revision ID: 0004_ocr_fts
+Revises: 0003_pgvector_embeddings
+Create Date: 2026-04-10
+
+Create ocr_text table for storing OCR results. Add a tsvector column
+to photos for unified full-text search (filename + user_title +
+user_notes) with a GIN index. OCR text is rolled up into a materialized
+view or joined at query time.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0004_ocr_fts"
+down_revision: Union[str, None] = "0003_pgvector_embeddings"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ── ocr_text table ────────────────────────────────────────────────
+    op.execute("""
+        CREATE TABLE IF NOT EXISTS ocr_text (
+            id VARCHAR PRIMARY KEY,
+            photo_id VARCHAR NOT NULL REFERENCES photos(id) ON DELETE CASCADE,
+            text TEXT NOT NULL,
+            language VARCHAR(8) DEFAULT '',
+            confidence FLOAT,
+            bbox JSONB,
+            created_at TIMESTAMPTZ DEFAULT now()
+        )
+    """)
+    op.execute("CREATE INDEX IF NOT EXISTS ix_ocr_text_photo_id ON ocr_text(photo_id)")
+
+    # ── tsvector column on photos ─────────────────────────────────────
+    op.execute("ALTER TABLE photos ADD COLUMN IF NOT EXISTS search_vector tsvector")
+    op.execute("CREATE INDEX IF NOT EXISTS ix_photos_search_vector ON photos USING GIN (search_vector)")
+
+    # Trigger to auto-update search_vector on INSERT/UPDATE
+    op.execute("""
+        CREATE OR REPLACE FUNCTION photos_search_vector_update() RETURNS trigger AS $$
+        BEGIN
+            NEW.search_vector :=
+                setweight(to_tsvector('english', coalesce(NEW.filename, '')), 'A') ||
+                setweight(to_tsvector('english', coalesce(NEW.user_title, '')), 'A') ||
+                setweight(to_tsvector('english', coalesce(NEW.user_notes, '')), 'B');
+            RETURN NEW;
+        END
+        $$ LANGUAGE plpgsql;
+    """)
+    op.execute("""
+        DO $$
+        BEGIN
+            IF NOT EXISTS (
+                SELECT 1 FROM pg_trigger WHERE tgname = 'photos_search_vector_trigger'
+            ) THEN
+                CREATE TRIGGER photos_search_vector_trigger
+                BEFORE INSERT OR UPDATE OF filename, user_title, user_notes
+                ON photos
+                FOR EACH ROW
+                EXECUTE FUNCTION photos_search_vector_update();
+            END IF;
+        END $$;
+    """)
+
+    # Backfill existing rows
+    op.execute("""
+        UPDATE photos SET search_vector =
+            setweight(to_tsvector('english', coalesce(filename, '')), 'A') ||
+            setweight(to_tsvector('english', coalesce(user_title, '')), 'A') ||
+            setweight(to_tsvector('english', coalesce(user_notes, '')), 'B')
+    """)
+
+
+def downgrade() -> None:
+    op.execute("DROP TRIGGER IF EXISTS photos_search_vector_trigger ON photos")
+    op.execute("DROP FUNCTION IF EXISTS photos_search_vector_update()")
+    op.execute("DROP INDEX IF EXISTS ix_photos_search_vector")
+    op.execute("ALTER TABLE photos DROP COLUMN IF EXISTS search_vector")
+    op.execute("DROP TABLE IF EXISTS ocr_text")