fix(fc3i): single-line celery.signals import + INT32 bounds on target_id + dict.fromkeys + rewrite retry test as direct _finalize unit

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_api_system_activity.py

@@ -45,7 +45,7 @@ def _reset_caches(monkeypatch):
 async def test_queues_returns_all_known_queues(client, monkeypatch):
     def _fake():
         return {
-            "queues": {q: 0 for q in activity_module._QUEUE_NAMES},
+            "queues": dict.fromkeys(activity_module._QUEUE_NAMES, 0),
             "fetched_at": datetime.now(UTC).isoformat(),
         }
     monkeypatch.setattr(activity_module, "_read_queues_sync", _fake)
@@ -75,7 +75,7 @@ async def test_queues_cached_within_ttl(client, monkeypatch):
 async def test_queues_redis_failure_returns_null_per_failing_queue(client, monkeypatch):
     """_read_queues_sync's per-queue try/except returns None on failure."""
     def _partial():
-        out = {q: 0 for q in activity_module._QUEUE_NAMES}
+        out = dict.fromkeys(activity_module._QUEUE_NAMES, 0)
         out["ml"] = None  # simulate broker hiccup on the ml queue
         return {"queues": out, "fetched_at": "x"}
     monkeypatch.setattr(activity_module, "_read_queues_sync", _partial)