refactor: remove dead code (safe cleanup)

- main.py: remove duplicate sqlalchemy select import
- main.py: remove unused json import in _telegram_poll_loop
- runtime.py: remove unreachable return after raise LockTimeoutError
- runtime.py: remove _restart_rdp_slot_bg (defined but never called)

app/runtime.py

@@ -701,27 +701,6 @@ def disconnect_rdp_slot(slot_id: int) -> None:
         db.close()
 
 
-def _restart_rdp_slot_bg(slot_id: int) -> None:
-    db = SessionLocal()
-    try:
-        slot = db.get(RdpSlot, slot_id)
-        if not slot or not slot.container_name:
-            return
-        service = db.get(Service, slot.service_id)
-        if not service:
-            return
-        try:
-            d = docker_client()
-            c = d.containers.get(slot.container_name)
-            c.restart(timeout=10)
-            logger.info("rdp_slot_container_restarted slot_id=%s", slot_id)
-        except docker.errors.NotFound:
-            start_rdp_slot_container(slot, service)
-        except Exception:
-            logger.exception("rdp_slot_container_restart_failed slot_id=%s", slot_id)
-    finally:
-        db.close()
-
 
 def stop_runtime_container(container_id: Optional[str]) -> None:
     if not container_id:
@@ -1093,8 +1072,6 @@ def allocator_lock(db: Session, lock_id: int, timeout_seconds: Optional[float] =
                 time.sleep(max(0.01, poll_seconds))
             raise LockTimeoutError(f"advisory lock timeout lock_id={lock_id} timeout={timeout_seconds}")
 
-            return self_nonlocal
-
         def __exit__(self_nonlocal, exc_type, exc, tb):
             return False