refactor(channels): normalize at cache fill and eliminate frontend as-cast

- 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

backend/internal/service/channel_available.go

@@ -66,7 +66,7 @@ func (s *ChannelService) ListAvailable(ctx context.Context) ([]AvailableChannel,
 		}
 		sort.SliceStable(groups, func(i, j int) bool { return groups[i].Name < groups[j].Name })
 
-		normalizeBillingModelSource(ch)
+		ch.normalizeBillingModelSource()
 
 		out = append(out, AvailableChannel{
 			ID:                 ch.ID,