Remove category from status messages

Instead of including the string "Warning: You're still in testing mode", for example, we should let `warnings` be a key under messages.

(This was a suggestion from another potential of the consumer of the API, and was also inconvenient when styling the frontend.)

No file, test, or entry point is named. Trace where the backend constructs status messages and inspect the API response shape consumed by the frontend; confirm that warnings are represented under a messages.warnings key instead of embedded as a category in the text. Done means the response shape supports frontend styling without the 'Warning:' prefix and relevant tests pass.

Written by the indexing model from the issue text.