fix: 对齐 JWT 常量 gofmt，并补全 available groups 契约字段

golangci-lint 要求 const 块按类型对齐；分组 DTO 新增
long_context_pricing_enabled 后，契约夹具未同步导致比较失败。

backend/internal/pkg/xai/subscription_tier.go

@@ -12,7 +12,7 @@ import (
 const GrokQuotaSignalMaxAge = 24 * time.Hour
 
 const (
-	grok45ResponsesModel       = "grok-4.5"
+	grok45ResponsesModel             = "grok-4.5"
 	grokHeavyQuotaRequestLimit int64 = 8_300
 	grokHeavyQuotaTokenLimit   int64 = 53_000_000
 )

backend/internal/server/api_contract_test.go

@@ -364,6 +364,7 @@ func TestAPIContracts(t *testing.T) {
 						"daily_limit_usd": null,
 						"weekly_limit_usd": null,
 						"monthly_limit_usd": null,
+						"long_context_pricing_enabled": false,
 						"image_price_1k": null,
 						"image_price_2k": null,
 						"image_price_4k": null,