GitLab

- channel.go: convert normalizeBillingModelSource into a (*Channel) method for entity cohesion
- channel_service.go: normalize in populateChannelCache so every cache-backed reader (gateway, billing, future endpoints) sees the default; drop the duplicate fallback inside resolveMapping
- table: tighten Row with status?: ChannelStatus / billing_model_source?: BillingModelSource, remove the [key: string]: unknown index signature
- admin view: drop the `as ChannelStatus` / `as BillingModelSource` assertions and add statusStyleOf / billingSourceLabelOf helpers with runtime fallback so unseen values render as "-" instead of crashing