Opt-in to overage billing for the current day.

When a paid user hits their daily limit, they can call this endpoint to authorize $0.50/request overage charges for the rest of the day. Sets a Redis flag that the auth middleware checks to allow requests through.

Requirements:

• Must have an active paid subscription (pro or premium)
• Must have a payment method on file (Stripe customer)

The overage authorization expires at the end of the UTC day. Overage usage is flushed to Stripe periodically and billed on the next invoice.