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/router.py

@@ -0,0 +1,22 @@
+from fastapi import APIRouter
+
+from app.plugins.auth.api.router import router as auth_router
+
+from .routers import artifacts, channels, mcp, models, skills, uploads
+from .routers.agents import router as agents_router
+from .routers.langgraph import feedback_router, runs_router, suggestion_router, threads_router
+
+router = APIRouter()
+
+router.include_router(auth_router)
+router.include_router(threads_router, prefix="/api/threads")
+router.include_router(runs_router, prefix="/api/threads")
+router.include_router(feedback_router, prefix="/api/threads")
+router.include_router(suggestion_router)
+router.include_router(agents_router)
+router.include_router(channels.router)
+router.include_router(artifacts.router)
+router.include_router(mcp.router)
+router.include_router(models.router)
+router.include_router(skills.router)
+router.include_router(uploads.router)