feat: raise LLM event limit to 200 and show total count awareness

- Bump LLM_MAX_EVENTS default from 50 to 200
- Add total_matched count to /api/ask response
- Include 'Showing X of Y total' header in LLM prompt so the model
  knows when its view is a subset and avoids false certainty
- Update system prompt to instruct acknowledging scale when truncated
- Update test mocks to accept new total parameter

backend/tests/test_ask.py

@@ -236,7 +236,7 @@ class TestAskEndpoint:
             }
         )
 
-        async def fake_llm(question, events):
+        async def fake_llm(question, events, total=None):
             return "The device had a failed wipe attempt."
 
         monkeypatch.setattr("routes.ask.LLM_API_KEY", "fake-key")
@@ -265,7 +265,7 @@ class TestAskEndpoint:
             }
         )
 
-        async def failing_llm(question, events):
+        async def failing_llm(question, events, total=None):
             raise RuntimeError("LLM service down")
 
         monkeypatch.setattr("routes.ask.LLM_API_KEY", "fake-key")