Delete a mismatch observation

Permanently removes the row (timestamp hard delete). This is separate from is_disabled, which only pauses processing. After delete, the observation no longer appears in GET list and cannot be PATCHed. Same permission as create/list/update. Returns 404 if the id is unknown, belongs to another customer, or is already deleted. Returns 404 if the id is unknown, belongs to another customer, or is already deleted.