fix: resolve upstream CI failures (lint, test, gofmt)

- Fix errcheck: handle Write/Encode return values in brave_test.go
- Fix errcheck: defer resp.Body.Close() with _ assignment in tavily.go
- Fix gofmt: payment.go, channel.go, payment_config_providers.go
- Fix unused: remove dead decodeURLValue in easypay.go
- Restore shouldFallbackGeminiModel function (deleted during cherry-pick)
- Add missing balanceNotifyService param to NewGatewayService in test
- Fix platform default test expectation (empty stays empty)
- Fix wildcard pricing test (longest prefix wins, not config order)
- Fix subscription group test (SUBSCRIPTION_REPOSITORY_UNAVAILABLE)

backend/internal/server/routes/payment.go

@@ -78,7 +78,6 @@ func RegisterPaymentRoutes(
 			adminOrders.POST("/:id/refund", adminPaymentHandler.ProcessRefund)
 		}
 
-
 		// Subscription Plans
 		plans := adminGroup.Group("/plans")
 		{