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/routers/api.py

@@ -125,7 +125,9 @@ def search(q: str, n: Optional[int] = 5, t: Optional[SearchType] = None, r: Opti
 
 @api.get('/update')
 def update(t: Optional[SearchType] = None, force: Optional[bool] = False):
+    state.search_index_lock.acquire()
     state.model = configure_search(state.model, state.config, regenerate=force, t=t)
+    state.search_index_lock.release()
     logger.info("Search Index updated via API call")
 
     return {'status': 'ok', 'message': 'index updated'}