[v0l]

Fixes #145

A balance a customer tops up and spends against purchases, renewals and
upgrades, in the same flow as NWC.

• POST /api/v1/account/credit/topup issues a Lightning invoice; on
  settlement the balance rises and a topup ledger row is written.
• GET/PATCH /api/v1/account/credit return and toggle the balance and
  the auto_apply opt-in.
• With auto_apply on, every quote applies min(balance, bill) and the
  external invoice is for the remainder. A bill fully covered by credit
  settles immediately with no invoice to pay.
• Auto-renewal draws on credit before saved payment methods, so a
  wallet-free account renews itself while the balance lasts.
• Admin GET/PATCH /api/admin/v1/users/{id}/credit and the ledger
  endpoint, gated by the new account_credit RBAC resource.

Credit covers net + tax, never the provider processing fee, and a balance
never goes negative: account_credit_event records every movement
(topup/spend/refund/admin) and the balance is the sum of the ledger.
Debits are written unsettled at invoice time and settled in
apply_payment next to discount redemptions, so every settlement path
records them exactly once.

Tests: 10 subscription unit tests, 11 mock DB tests, 5 e2e scenarios
(passing against the docker stack).