feat: add Alembic for database migrations

Replace inline migration logic with proper Alembic migrations:
- 001: Initial schema (creates memories table with FTS)
- 002: Add multi-user and secrets columns (user_id, is_sensitive,
  vault_path, encrypted_content)

Migrations run automatically on app startup. Existing databases
are handled gracefully with IF NOT EXISTS / column existence checks.

pyproject.toml

@@ -16,7 +16,7 @@ classifiers = [
 ]
 
 [project.optional-dependencies]
-api = ["fastapi>=0.115", "asyncpg>=0.30", "uvicorn>=0.34", "pydantic>=2.0"]
+api = ["fastapi>=0.115", "asyncpg>=0.30", "uvicorn>=0.34", "pydantic>=2.0", "alembic>=1.14", "sqlalchemy>=2.0"]
 vault = ["hvac>=2.0"]
 dev = ["pytest>=8.0", "pytest-asyncio>=0.24", "ruff>=0.8", "mypy>=1.13", "httpx>=0.28", "cryptography>=43.0"]