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/alembic/versions/0001_baseline.py

@@ -0,0 +1,27 @@
+"""baseline (empty)
+
+Revision ID: 0001_baseline
+Revises:
+Create Date: 2026-04-10
+
+The current schema is created by SQLAlchemy `Base.metadata.create_all` in
+`app.database.init_db()` on first boot. Alembic only owns deltas from
+PR3 onward. This baseline is intentionally empty so `alembic upgrade head`
+on a fresh DB simply creates the `alembic_version` table and stamps it.
+"""
+from typing import Sequence, Union
+
+
+# revision identifiers, used by Alembic.
+revision: str = "0001_baseline"
+down_revision: Union[str, None] = None
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    pass
+
+
+def downgrade() -> None:
+    pass