fix: add proper error handling for AI generation and log failures

Made-with: Cursor

custom_addons/encoach_ai/controllers/ai_controller.py

@@ -347,7 +347,8 @@ class AIController(http.Controller):
             ]
             return _json_response(ai.chat_json(messages, action=f"exam_generate_{module}"))
         except Exception as e:
-            return _json_response({"questions": [], "error": str(e)})
+            _logger.exception("exam_generate %s failed: %s", module, e)
+            return _json_response({"questions": [], "error": str(e)}, 500)
 
     def _generate_passage(self, ai, body):
         topic = body.get("topic", "general knowledge")