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/env.py

@@ -0,0 +1,96 @@
+"""
+Alembic environment for PhotoVault.
+
+Pulls DATABASE_URL from the environment so the same migrations work in
+docker compose and locally. Strips the async driver suffix because Alembic
+runs synchronously via psycopg2.
+
+Future-migration note
+---------------------
+Fresh installs run `Base.metadata.create_all` in `app.database.init_db`
+*before* migrations would normally apply, so any migration that adds a
+column / index / table to an object the model already declares will see
+that object already present. Write migrations defensively:
+
+    op.execute("ALTER TABLE photos ADD COLUMN IF NOT EXISTS new_col TEXT")
+    op.execute("CREATE INDEX IF NOT EXISTS ix_foo ON foo(bar)")
+
+For brand-new tables that the model also declares, the same applies — use
+`op.execute("CREATE TABLE IF NOT EXISTS ...")` or check first.
+"""
+from logging.config import fileConfig
+import os
+import sys
+from pathlib import Path
+
+from sqlalchemy import engine_from_config, pool
+from alembic import context
+
+# Make `app` importable from this script.
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+
+from app.database import Base  # noqa: E402
+# Import all models so they're registered on Base.metadata for autogenerate.
+from app.models import (  # noqa: E402, F401
+    Photo,
+    Folder,
+    SourceRoot,
+    Tag,
+    Heap,
+    HeapPhoto,
+    Embedding,
+)
+
+config = context.config
+
+# Resolve DATABASE_URL from env. Strip async driver suffixes — Alembic
+# uses sync drivers.
+db_url = os.environ.get("DATABASE_URL") or config.get_main_option("sqlalchemy.url")
+if db_url:
+    if "+asyncpg" in db_url:
+        db_url = db_url.replace("+asyncpg", "+psycopg2")
+    elif db_url.startswith("postgresql://"):
+        db_url = db_url.replace("postgresql://", "postgresql+psycopg2://", 1)
+    elif "+aiosqlite" in db_url:
+        db_url = db_url.replace("+aiosqlite", "")
+    config.set_main_option("sqlalchemy.url", db_url)
+
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = Base.metadata
+
+
+def run_migrations_offline() -> None:
+    """Run migrations in 'offline' mode (emit SQL only)."""
+    url = config.get_main_option("sqlalchemy.url")
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    """Run migrations against a live database."""
+    connectable = engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+    with connectable.connect() as connection:
+        context.configure(
+            connection=connection,
+            target_metadata=target_metadata,
+        )
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()