Remove Payment Method

Cancel the user's active subscription and remove the stored payment method.

This endpoint is retained for frontend compatibility, but its behavior is subscription-centric:

1. Cancels the user's active Stripe subscription immediately
2. Downgrades the user to the Hobby plan in Supabase
3. Removes the stored card from the Stripe customer

Args: request: Active subscription payment method ID shown in the UI auth: Authenticated user context

Returns: Success status with cancellation message