Hard-delete a single test case by ID.

Permanently removes a test case and all its child entities (payments, chats, files, timeline entries, webhook delivery records) from the database.

This is a hard-delete. The row is removed entirely — there is no soft-delete or recovery path. This is intentional for CI environments that create many test cases per day.

Guards:

• Returns 400 if the case is classified as Production. Only Test cases may be deleted via this endpoint.
• Returns 404 if the case is not found or not owned by the calling creditor.

Idempotent: Returns 204 even if the case is already deleted (no-op).