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/config.py

@@ -46,7 +46,7 @@ class Settings(BaseSettings):
     LLM_API_KEY: str = ""
     LLM_BASE_URL: str = "https://api.openai.com/v1"
     LLM_MODEL: str = "gpt-4o-mini"
-    LLM_MAX_EVENTS: int = 50
+    LLM_MAX_EVENTS: int = 200
     LLM_TIMEOUT_SECONDS: int = 30
     LLM_API_VERSION: str = ""  # e.g. 2025-01-01-preview for Azure OpenAI