Add async lock protection to geo service cache and mark Task 16 done

backend/app/tasks/geo_re_resolve.py

@@ -67,7 +67,7 @@ async def _run_re_resolve_with_resources(settings: "Settings", http_session: "Cl
         log.info("geo_re_resolve_start", unresolved=len(unresolved_ips))
 
         # Clear the negative cache so these IPs are eligible for fresh API calls.
-        geo_service.clear_neg_cache()
+        await geo_service.clear_neg_cache()
 
         # lookup_batch handles throttling, retries, and persistence when db is
         # passed.  This is a background task so DB writes are allowed.