revert: remove antigravity credits precheck logic (not part of channel feature)

Restore account_usage_service.go, antigravity_gateway_service.go,
antigravity_credits_overages.go and its test to upstream/main state.
These credits balance precheck changes were accidentally included
during cherry-pick of channel management commits.

backend/internal/service/antigravity_gateway_service.go

@@ -557,13 +557,7 @@ func (s *AntigravityGatewayService) antigravityRetryLoop(p antigravityRetryLoopP
 	if p.requestedModel != "" && p.account.Platform == PlatformAntigravity &&
 		p.account.IsOveragesEnabled() && !p.account.isCreditsExhausted() &&
 		p.account.isModelRateLimitedWithContext(p.ctx, p.requestedModel) {
-		// Check actual credits balance before injection
-		if !s.checkAccountCredits(p.ctx, p.account) {
-			// No credits available - mark as exhausted and skip injection
-			s.setCreditsExhausted(p.ctx, p.account)
-			logger.LegacyPrintf("service.antigravity_gateway", "%s pre_check: no_credits_available account=%d (skipping credits injection)",
-				p.prefix, p.account.ID)
-		} else if creditsBody := injectEnabledCreditTypes(p.body); creditsBody != nil {
+		if creditsBody := injectEnabledCreditTypes(p.body); creditsBody != nil {
 			p.body = creditsBody
 			overagesInjected = true
 			logger.LegacyPrintf("service.antigravity_gateway", "%s pre_check: model_rate_limited_credits_inject model=%s account=%d (injecting enabledCreditTypes)",
@@ -876,15 +870,14 @@ func logPrefix(sessionID, accountName string) string {
 
 // AntigravityGatewayService 处理 Antigravity 平台的 API 转发
 type AntigravityGatewayService struct {
-	accountRepo         AccountRepository
-	tokenProvider       *AntigravityTokenProvider
-	rateLimitService    *RateLimitService
-	httpUpstream        HTTPUpstream
-	settingService      *SettingService
-	cache               GatewayCache // 用于模型级限流时清除粘性会话绑定
-	schedulerSnapshot   *SchedulerSnapshotService
-	internal500Cache    Internal500CounterCache // INTERNAL 500 渐进惩罚计数器
-	accountUsageService *AccountUsageService    // 共享 usage 缓存，用于积分余额检查
+	accountRepo       AccountRepository
+	tokenProvider     *AntigravityTokenProvider
+	rateLimitService  *RateLimitService
+	httpUpstream      HTTPUpstream
+	settingService    *SettingService
+	cache             GatewayCache // 用于模型级限流时清除粘性会话绑定
+	schedulerSnapshot *SchedulerSnapshotService
+	internal500Cache  Internal500CounterCache // INTERNAL 500 渐进惩罚计数器
 }
 
 func NewAntigravityGatewayService(
@@ -896,18 +889,16 @@ func NewAntigravityGatewayService(
 	httpUpstream HTTPUpstream,
 	settingService *SettingService,
 	internal500Cache Internal500CounterCache,
-	accountUsageService *AccountUsageService,
 ) *AntigravityGatewayService {
 	return &AntigravityGatewayService{
-		accountRepo:         accountRepo,
-		tokenProvider:       tokenProvider,
-		rateLimitService:    rateLimitService,
-		httpUpstream:        httpUpstream,
-		settingService:      settingService,
-		cache:               cache,
-		schedulerSnapshot:   schedulerSnapshot,
-		internal500Cache:    internal500Cache,
-		accountUsageService: accountUsageService,
+		accountRepo:       accountRepo,
+		tokenProvider:     tokenProvider,
+		rateLimitService:  rateLimitService,
+		httpUpstream:      httpUpstream,
+		settingService:    settingService,
+		cache:             cache,
+		schedulerSnapshot: schedulerSnapshot,
+		internal500Cache:  internal500Cache,
 	}
 }