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/app/config.py

@@ -39,9 +39,11 @@ class MulitaConfig(BaseModel):
 
 class Settings(BaseSettings):
     """Application settings"""
-    # Database
+    # Database — Postgres + pgvector by default. The SQLite escape hatch
+    # remains supported via the docker-compose.sqlite.yml override and by
+    # setting DATABASE_URL=sqlite+aiosqlite:///... in .env for local dev.
     database_url: str = Field(
-        default="sqlite+aiosqlite:///data/db/mulita.db",
+        default="postgresql+asyncpg://mulita:mulita@db:5432/mulita",
         env="DATABASE_URL"
     )