collab: Backfill github_user_created_at on users (#16600)

This PR adds a backfiller to backfill the `github_user_created_at`
column on users.

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -682,6 +682,7 @@ impl TestServer {
                 supermaven_admin_api_key: None,
                 qwen2_7b_api_key: None,
                 qwen2_7b_api_url: None,
+                user_backfiller_github_access_token: None,
             },
         })
     }