refactor(gateway): restructure gateway with new dependency injection and services

Reorganize app/gateway/ with:
- common/ - lifespan management
- dependencies/ - FastAPI dependency injection (db, checkpointer, repositories, stream_bridge)
- services/runs/ - run execution services (facade_factory, input adapters, store operations)
- registrar.py - router registration
- router.py - main router setup

Simplify app.py to use the new modular structure.
Remove deprecated utils.py.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/gateway/services/runs/storage_observer.py

@@ -0,0 +1,5 @@
+"""Compatibility wrapper for the app-owned storage observer."""
+
+from app.infra.storage.runs import StorageRunObserver
+
+__all__ = ["StorageRunObserver"]