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.sqlite.yml

@@ -0,0 +1,47 @@
+# SQLite escape hatch override.
+#
+# Usage (omit the `db` service from the up command):
+#
+#   docker compose -f docker-compose.yml -f docker-compose.sqlite.yml \
+#       up frontend backend worker redis
+#
+# This pins the backend and worker to the legacy SQLite database file at
+# /data/db/mulita.db (in the existing db_data volume), drops the dependency
+# on Postgres, and skips Alembic — the SQLite schema is still managed by
+# the inline ALTERs in app/database.py:init_db.
+#
+# Vision features that depend on pgvector (PR4 onward) will refuse to enable
+# in this mode; the search/embedding endpoints will return 503 with a clear
+# error pointing back at the default Postgres setup.
+
+services:
+  backend:
+    command: sh -c "uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload"
+    environment:
+      - DATABASE_URL=sqlite+aiosqlite:////data/db/mulita.db
+      - REDIS_URL=redis://redis:6379
+      - CELERY_BROKER_URL=redis://redis:6379
+      - CELERY_RESULT_BACKEND=redis://redis:6379
+      - PHOTO_DIRS=${PHOTO_DIRS:-/photos}
+      - ALLOWED_ORIGINS=${ALLOWED_ORIGINS:-*}
+      - LOG_LEVEL=${LOG_LEVEL:-INFO}
+      - TZ=${TZ:-UTC}
+    depends_on:
+      redis:
+        condition: service_started
+
+  worker:
+    environment:
+      - DATABASE_URL=sqlite+aiosqlite:////data/db/mulita.db
+      - REDIS_URL=redis://redis:6379
+      - CELERY_BROKER_URL=redis://redis:6379
+      - CELERY_RESULT_BACKEND=redis://redis:6379
+      - PHOTO_DIRS=${PHOTO_DIRS:-/photos}
+      - CELERYD_CONCURRENCY=${CELERYD_CONCURRENCY:-4}
+      - LOG_LEVEL=${LOG_LEVEL:-INFO}
+      - TZ=${TZ:-UTC}
+    depends_on:
+      redis:
+        condition: service_started
+      backend:
+        condition: service_started