Collapse blockquote and strip <think> blocks from AI responses

- blockquote expandable: replies are collapsed by default in Telegram
- strip <think>...</think> sections (chain-of-thought from reasoning
  models like DeepSeek R1 / QwQ) before sending the response

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

app/ai/service.py

@@ -626,12 +626,17 @@ class AIService:
             return ""
         content = getattr(message, "content", None)
         if isinstance(content, str):
-            return content.strip()
+            return self._strip_think(content)
         if isinstance(content, list):
             parts = []
             for part in content:
                 text = getattr(part, "text", None)
                 if isinstance(text, str):
                     parts.append(text)
-            return "\n".join(parts).strip()
+            return self._strip_think("\n".join(parts))
         return ""
+
+    @staticmethod
+    def _strip_think(text: str) -> str:
+        import re
+        return re.sub(r"<think>.*?</think>", "", text, flags=re.DOTALL).strip()