ref(heartbeat): Remove cron provisioning

Remove gateway cron helpers and startup provisioning, relying on\nheartbeat instructions instead. Also drop unused httpx dependency.\n\nCo-Authored-By: Claude <noreply@anthropic.com>

backend/app/main.py

@@ -12,7 +12,6 @@ from app.api.tasks import router as tasks_router
 from app.core.config import settings
 from app.core.logging import configure_logging
 from app.db.session import init_db
-from app.services.cron_jobs import ensure_mission_control_cron_job
 
 configure_logging()
 
@@ -30,9 +29,8 @@ if origins:
 
 
 @app.on_event("startup")
-async def on_startup() -> None:
+def on_startup() -> None:
     init_db()
-    await ensure_mission_control_cron_job()
 
 
 @app.get("/health")