refactor(channels): centralize BillingModelSource normalization and exhaustive enum maps

- service: add normalizeBillingModelSource helper, apply in Create/GetByID/Update/List/ListAvailable outputs
- handler: drop channelToResponse fallback now that service owns the default; add passthrough test
- frontend: replace ternary status/billing-source lookups with Record<Enum, ...> maps so new union members fail the build
- chip/table: drop local type aliases, reuse UserSupportedModel/UserPricingInterval directly
- tests: assert short-circuit on ListAll error, wrap-prefix preservation, and Name-based default lookup

backend/internal/handler/admin/channel_handler.go

@@ -158,9 +158,6 @@ func channelToResponse(ch *service.Channel) *channelResponse {
 		UpdatedAt:      ch.UpdatedAt.Format("2006-01-02T15:04:05Z"),
 	}
 	resp.BillingModelSource = ch.BillingModelSource
-	if resp.BillingModelSource == "" {
-		resp.BillingModelSource = service.BillingModelSourceChannelMapped
-	}
 	if resp.GroupIDs == nil {
 		resp.GroupIDs = []int64{}
 	}