Fix unnatural response cutoffs

- Replace aggressive sentence-count limiting with ensure_complete_thought()
  which only trims if the LLM was actually cut off mid-sentence
- Softer prompt guidance for natural brevity instead of rigid sentence count
- max_tokens at 100 as natural length cap

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/services/llm.py

@@ -124,7 +124,7 @@ class LLMService:
                         json={
                             "model": self.openrouter_model,
                             "messages": messages,
-                            "max_tokens": 150,
+                            "max_tokens": 100,
                         },
                     )
                     response.raise_for_status()