test(app): add split app.py tests and extend migration_engine coverage
- app.py split from 1966-line test_app.py into 7 files (68 tests, 92% coverage):
- test_app_lifespan: lifespan, ensure_initial_admin_user
- test_app_handlers: exception handlers, HSTS
- test_app_middleware: log_requests, middleware chain
- test_app_ws_auth: _authenticate_websocket
- test_app_ws_endpoint: WS main loop, 5 endpoint handlers
- test_app_ws_events: task/maintenance/dataset/translate WS streams
- test_app_spa: SPA serving, TestClient integration
- migration_engine: extended coverage with init, edge cases, error paths
busya
f87c873027