refactor(events): drop forward-ref quote and test triggered_by default

Code review follow-up on f4d66dd:
- _handle_arm_request signature used "ArmStateFSM" as a string forward
  reference even though the type is imported at module top.
  _handle_event uses the bare form; match it for consistency.
- Add a test asserting that omitting triggered_by in an arm-request
  payload defaults to "unknown". That value feeds the audit log, so
  it deserves explicit regression coverage.

No behavior change.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_events.py

@@ -480,3 +480,25 @@ class TestArmRequestDispatch:
         )
 
         assert calls == []
+
+    def test_arm_request_default_triggered_by(self, test_db):
+        """Omitting triggered_by must default to 'unknown' (audit-log value)."""
+        from vigilar.events.processor import EventProcessor
+
+        processor = EventProcessor.__new__(EventProcessor)
+        calls = []
+
+        class FakeFSM:
+            state = ArmState.DISARMED
+
+            def transition(self, new_state, pin="", triggered_by="system"):
+                calls.append((new_state, pin, triggered_by))
+                return True
+
+        processor._handle_arm_request(
+            payload={"mode": "DISARMED", "pin": ""},
+            fsm=FakeFSM(),
+        )
+
+        assert len(calls) == 1
+        assert calls[0][2] == "unknown"