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

.env.example

@@ -42,6 +42,6 @@ ALERTS_ENABLED=false
 LLM_API_KEY=
 LLM_BASE_URL=https://api.openai.com/v1
 LLM_MODEL=gpt-4o-mini
-LLM_MAX_EVENTS=50
+LLM_MAX_EVENTS=200
 LLM_TIMEOUT_SECONDS=30
 LLM_API_VERSION=