Run Phone Billing Endpoint

Run monthly recurring billing for phone numbers.

Intended to run daily via Cloud Scheduler. The logic is idempotent per day (already-charged numbers skip). Handles the full lifecycle:

1. Numbers where next_billing_at <= NOW() and status='active' without grace:

   • Try to deduct monthly_price_usd from wallet.
   • Success: advance next_billing_at +30d, log 'charged'.
   • Fail (insufficient balance): start 10-day grace, suspend_at=+3d, send warning email.

2. Numbers in grace (grace_ends_at IS NOT NULL) not yet released:

   • Retry charge every day. Success → reactivate (clear grace, advance next_billing_at, send reactivation email).
   • If suspended_at passed and status='active': flip to 'suspended', send suspended email.

3. Numbers where grace_ends_at <= NOW() and still unpaid:

   • Release at the carrier that holds it (Telnyx or Twilio), mark phone_numbers.status='inactive', send released email.

Scope: provider IN ('telnyx', 'twilio') — the carriers we rent from and resell. sip and netgsm rows are deliberately out of scope: it is not established that we carry a monthly cost for those, and several look like customer-supplied trunks, where charging rent would be wrong.