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>

docker-compose.yml

@@ -1,5 +1,3 @@
-version: '3.8'
-
 services:
   frontend:
     build:
@@ -36,9 +34,13 @@ services:
       - ${PHOTO_DIRS:-./photos}:/photos:rw
       - thumbs_data:/data/thumbs
       - proxies_data:/data/proxies
-      - db_data:/data/db
+      - db_data:/data/db   # retained so the docker-compose.sqlite.yml override has somewhere to put mulita.db
+    # Run Alembic migrations before starting uvicorn. On a fresh Postgres
+    # the empty 0001 baseline is a no-op stamp; create_all in init_db then
+    # builds the schema.
+    command: sh -c "alembic upgrade head && uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload"
     environment:
-      - DATABASE_URL=sqlite+aiosqlite:////data/db/mulita.db
+      - DATABASE_URL=postgresql+asyncpg://mulita:mulita@db:5432/mulita
       - REDIS_URL=redis://redis:6379
       - CELERY_BROKER_URL=redis://redis:6379
       - CELERY_RESULT_BACKEND=redis://redis:6379
@@ -47,7 +49,10 @@ services:
       - LOG_LEVEL=${LOG_LEVEL:-INFO}
       - TZ=${TZ:-UTC}
     depends_on:
-      - redis
+      redis:
+        condition: service_started
+      db:
+        condition: service_healthy
     networks:
       - mulita-network
     restart: unless-stopped
@@ -65,7 +70,7 @@ services:
       - proxies_data:/data/proxies
       - db_data:/data/db
     environment:
-      - DATABASE_URL=sqlite+aiosqlite:////data/db/mulita.db
+      - DATABASE_URL=postgresql+asyncpg://mulita:mulita@db:5432/mulita
       - REDIS_URL=redis://redis:6379
       - CELERY_BROKER_URL=redis://redis:6379
       - CELERY_RESULT_BACKEND=redis://redis:6379
@@ -74,12 +79,34 @@ services:
       - LOG_LEVEL=${LOG_LEVEL:-INFO}
       - TZ=${TZ:-UTC}
     depends_on:
-      - redis
-      - backend
+      redis:
+        condition: service_started
+      backend:
+        condition: service_started
+      db:
+        condition: service_healthy
     networks:
       - mulita-network
     restart: unless-stopped
 
+  db:
+    image: pgvector/pgvector:pg16
+    container_name: mulita-db
+    environment:
+      POSTGRES_USER: mulita
+      POSTGRES_PASSWORD: mulita
+      POSTGRES_DB: mulita
+    volumes:
+      - pg_data:/var/lib/postgresql/data
+    networks:
+      - mulita-network
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U mulita -d mulita"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+
   redis:
     image: redis:7-alpine
     container_name: mulita-redis
@@ -102,4 +129,5 @@ volumes:
   thumbs_data:
   proxies_data:
   db_data:
-  redis_data:
+  redis_data:
+  pg_data: