fix: better error handling for chat AI errors

Catch all exceptions (not just ValueError) and return meaningful
error messages from OpenAI API errors instead of generic 500.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/routers/chat.py

@@ -97,6 +97,15 @@ async def send_message(
     except ValueError as exc:
         # AI not configured
         raise HTTPException(status_code=status.HTTP_422_UNPROCESSABLE_ENTITY, detail=str(exc))
+    except Exception as exc:
+        error_msg = str(exc)
+        # Extract meaningful error from OpenAI exceptions
+        if hasattr(exc, 'message'):
+            error_msg = exc.message
+        elif hasattr(exc, 'body') and isinstance(exc.body, dict):
+            error_msg = exc.body.get('error', {}).get('message', error_msg)
+        logger.error("Chat error: %s", error_msg)
+        raise HTTPException(status_code=500, detail=f"AI error: {error_msg[:500]}")
     except Exception as exc:
         logger.exception("Chat error for user %s", user.id)
         raise HTTPException(