feat(auth): implement API key authentication and management

Introduce a new API key authentication mechanism to support service-to-service
communication (e.g., Airflow, CI/CD) without browser-based JWT login.

- Add `APIKey` model and `APIKeyPrincipal` dependency for RBAC.
- Implement `require_api_key_or_jwt` dependency with environment scoping.
- Add admin CRUD endpoints for API key lifecycle management.
- Add API key management UI in System Settings.
- Update maintenance API to support explicit `environment_id` and API key auth.
- Add i18n support for API keys and connection settings.
- Include Python and Bash usage examples in the `examples/` directory.
- Update technical specifications and documentation.

backend/src/core/database.py

@@ -15,6 +15,7 @@ from sqlalchemy.orm import sessionmaker
 
 # Import models to ensure they're registered with Base
 from ..models import (
+    api_key as _api_key_models,  # noqa: F401
     assistant as _assistant_models,  # noqa: F401
     auth as _auth_models,  # noqa: F401
     clean_release as _clean_release_models,  # noqa: F401