Lock the index before updating it via API or Scheduler

- There are 3 paths to updating/setting the index (stored in state.model)
  - App start
  - API
  - Scheduler

- Put all updates to the index behind a lock. As multiple updates path
that could (potentially) run at the same time (via API or Scheduler)

src/utils/state.py

@@ -1,4 +1,5 @@
 # Standard Packages
+import threading
 from packaging import version
 
 # External Packages
@@ -20,6 +21,7 @@ host: str = None
 port: int = None
 cli_args: list[str] = None
 query_cache = LRU()
+search_index_lock = threading.Lock()
 
 if torch.cuda.is_available():
     # Use CUDA GPU