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/app.py

@@ -25,6 +25,7 @@ from starlette.middleware.sessions import SessionMiddleware
 from .api import auth
 from .api.routes import (
     admin,
+    admin_api_keys,
     assistant,
     clean_release,
     clean_release_v2,
@@ -224,6 +225,7 @@ app.add_middleware(TraceContextMiddleware)
 # Include API routes
 app.include_router(auth.router)
 app.include_router(admin.router)
+app.include_router(admin_api_keys.router)
 app.include_router(plugins.router, prefix="/api/plugins", tags=["Plugins"])
 app.include_router(tasks.router, prefix="/api/tasks", tags=["Tasks"])
 app.include_router(settings.router, prefix="/api/settings", tags=["Settings"])