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/service/account_stats_pricing_test.go

@@ -147,14 +147,14 @@ func TestFindPricingForModel(t *testing.T) {
 			wantNil: true,
 		},
 		{
-			name: "wildcard matches by config order (first match wins)",
+			name: "wildcard matches by longest prefix (most specific wins)",
 			list: []ChannelModelPricing{
 				{ID: 10, Models: []string{"claude-*"}},
 				{ID: 11, Models: []string{"claude-opus-*"}},
 			},
 			platform: "",
 			model:    "claude-opus-4",
-			wantID:   10, // config order: "claude-*" is first and matches, so it wins
+			wantID:   11, // "claude-opus-*" is longer prefix, wins over "claude-*"
 		},
 		{
 			name: "shorter wildcard used when longer does not match",