Address automated review feedback on the always-on assets change:
- setup_database(): on DB init failure or missing dependencies, call
disable_assets_routes() + asset_seeder.disable() so /api/assets/* returns a
clean 503 instead of 500s against an uninitialized DB (restores the fail-safe
the removed else-branch used to provide).
- feature_flags: derive the advertised 'assets' capability from live backend
availability instead of hardcoding True, so clients degrade gracefully.
- cli_args: re-add --enable-assets as a hidden deprecated no-op so existing
launchers still passing the flag don't abort argparse.
- routes: add assets_enabled() accessor for the feature-flag derivation.
b369532116