fix: remove /debug/exec endpoint that allows authenticated RCE via exec() (#2178)

The /api/v1/system/debug/exec endpoint passes user-supplied HTTP body
directly to Python exec(), enabling arbitrary code execution for any
authenticated user when debug_mode is enabled. This is a critical
security risk (CWE-94): a single misconfiguration or compromised JWT
grants full server-side code execution.

Remove the endpoint entirely. The /debug/plugin/action endpoint (which
does not use exec()) is left intact as it serves a different, scoped
purpose.

Co-authored-by: Junyan Chin <rockchinq@gmail.com>

src/langbot/pkg/api/http/controller/groups/system.py

@@ -140,17 +140,6 @@ class SystemRouterGroup(group.RouterGroup):
         async def _() -> str:
             return self.success(data=await self.ap.maintenance_service.get_storage_analysis())
 
-        @self.route('/debug/exec', methods=['POST'], auth_type=group.AuthType.USER_TOKEN)
-        async def _() -> str:
-            if not constants.debug_mode:
-                return self.http_status(403, 403, 'Forbidden')
-
-            py_code = await quart.request.data
-
-            ap = self.ap
-
-            return self.success(data=exec(py_code, {'ap': ap}))
-
         @self.route(
             '/debug/plugin/action',
             methods=['POST'],