phsl / new-api

Follow-up to #6518 (issue #6480) addressing three review findings:

- Document and lock in arrears semantics for the wallet Reserve top-up:
  when an auto-group retry lands on a more expensive group, the full
  reservation delta is deducted unconditionally (balance may go
  negative), mirroring settlement, so the logged pre-consumed quota
  always reconciles with the actual balance movement. Genuine DB
  errors still fail the attempt with update_data_error. Subscription
  funding keeps its insufficient-quota behavior: subscriptions enforce
  a hard used<=total cap and do not support arrears.
- PriceData.FreeModel is cleared when a retry switches from a free
  group to a paid one, keeping it consistent with the billing session
  created at that point.
- getChannel refreshes GroupRatioInfo only after channel selection
  succeeds, and the retry loop records the channel in use_channel
  before PrepareTieredBillingForSelectedGroup can fail.