feat: migrate to Postgres + pgvector with Alembic scaffolding

Switch the default database from SQLite to Postgres + pgvector (via
pgvector/pgvector:pg16 Docker image) to support the upcoming vision
pipeline (embeddings, OCR, object detection, face clustering).

- Add `db` service to docker-compose.yml with healthcheck
- Wire `alembic upgrade head` into backend CMD before uvicorn
- Bootstrap empty 0001_baseline revision (schema still owned by create_all)
- Guard SQLite-only PRAGMAs and inline ALTERs behind _is_sqlite flag
- Run `CREATE EXTENSION IF NOT EXISTS vector` on Postgres init
- Add asyncpg, psycopg2-binary, pgvector to requirements
- Provide docker-compose.sqlite.yml escape hatch for legacy SQLite mode

Fresh DB + rescan assumed — no SQLite→Postgres data migration.

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

backend/requirements.txt

@@ -5,7 +5,10 @@ python-multipart==0.0.6
 
 # Database
 sqlalchemy[asyncio]==2.0.25
-aiosqlite==0.19.0
+aiosqlite==0.19.0           # SQLite escape hatch (docker-compose.sqlite.yml override)
+asyncpg==0.29.0             # async Postgres driver (default)
+psycopg2-binary==2.9.9      # sync Postgres driver, used by Alembic CLI
+pgvector==0.2.5             # pgvector SQLAlchemy types
 alembic==1.13.1
 
 # Redis and Celery