Delete Account

Permanently delete the user's account and personal data.

Mirrors the expert-side delete endpoint. Behavior:

• Blocks if there's an active chat session (must close first).
• Cancels any active Stripe subscription (immediate, no proration).
• Deletes the Stripe Customer (removes saved payment methods, stops future invoicing).
• Revokes the API key (clears api_key + hash + prefix so any agent holding the old key can't make new requests).
• Best-effort deletes the Auth0 user so they can't log back in.
• Anonymizes the User row's identifying fields. The row itself is retained so CreditTransaction / ChatSession / Question records keep their FK integrity for audit purposes.

Returns 204 on success. The frontend should clear local auth state and redirect to the logged-out landing page.