feat(backend): implement gemini quota simulation and rate limiting

- feat: add local quota tracking for gemini tiers (Legacy/Pro/Ultra)
- feat: implement PreCheckUsage in RateLimitService
- feat: align gemini daily reset window with PST
- fix: sticky session fallback logic

backend/internal/service/account_usage_service.go

@@ -97,6 +97,8 @@ type UsageInfo struct {
 	FiveHour       *UsageProgress `json:"five_hour"`                  // 5小时窗口
 	SevenDay       *UsageProgress `json:"seven_day,omitempty"`        // 7天窗口
 	SevenDaySonnet *UsageProgress `json:"seven_day_sonnet,omitempty"` // 7天Sonnet窗口
+	GeminiProDaily *UsageProgress `json:"gemini_pro_daily,omitempty"` // Gemini Pro 日配额
+	GeminiFlashDaily *UsageProgress `json:"gemini_flash_daily,omitempty"` // Gemini Flash 日配额
 }
 
 // ClaudeUsageResponse Anthropic API返回的usage结构
@@ -122,17 +124,19 @@ type ClaudeUsageFetcher interface {
 
 // AccountUsageService 账号使用量查询服务
 type AccountUsageService struct {
-	accountRepo  AccountRepository
-	usageLogRepo UsageLogRepository
-	usageFetcher ClaudeUsageFetcher
+	accountRepo         AccountRepository
+	usageLogRepo        UsageLogRepository
+	usageFetcher        ClaudeUsageFetcher
+	geminiQuotaService  *GeminiQuotaService
 }
 
 // NewAccountUsageService 创建AccountUsageService实例
-func NewAccountUsageService(accountRepo AccountRepository, usageLogRepo UsageLogRepository, usageFetcher ClaudeUsageFetcher) *AccountUsageService {
+func NewAccountUsageService(accountRepo AccountRepository, usageLogRepo UsageLogRepository, usageFetcher ClaudeUsageFetcher, geminiQuotaService *GeminiQuotaService) *AccountUsageService {
 	return &AccountUsageService{
-		accountRepo:  accountRepo,
-		usageLogRepo: usageLogRepo,
-		usageFetcher: usageFetcher,
+		accountRepo:        accountRepo,
+		usageLogRepo:       usageLogRepo,
+		usageFetcher:       usageFetcher,
+		geminiQuotaService: geminiQuotaService,
 	}
 }
 
@@ -146,6 +150,10 @@ func (s *AccountUsageService) GetUsage(ctx context.Context, accountID int64) (*U
 		return nil, fmt.Errorf("get account failed: %w", err)
 	}
 
+	if account.Platform == PlatformGemini {
+		return s.getGeminiUsage(ctx, account)
+	}
+
 	// 只有oauth类型账号可以通过API获取usage（有profile scope）
 	if account.CanGetUsage() {
 		var apiResp *ClaudeUsageResponse
@@ -192,6 +200,33 @@ func (s *AccountUsageService) GetUsage(ctx context.Context, accountID int64) (*U
 	return nil, fmt.Errorf("account type %s does not support usage query", account.Type)
 }
 
+func (s *AccountUsageService) getGeminiUsage(ctx context.Context, account *Account) (*UsageInfo, error) {
+	now := time.Now()
+	start := geminiDailyWindowStart(now)
+
+	stats, err := s.usageLogRepo.GetModelStatsWithFilters(ctx, start, now, 0, 0, account.ID)
+	if err != nil {
+		return nil, fmt.Errorf("get gemini usage stats failed: %w", err)
+	}
+
+	usage := &UsageInfo{
+		UpdatedAt: &now,
+	}
+
+	quota, ok := s.geminiQuotaService.QuotaForAccount(ctx, account)
+	if !ok {
+		return usage, nil
+	}
+
+	totals := geminiAggregateUsage(stats)
+	resetAt := geminiDailyResetTime(now)
+
+	usage.GeminiProDaily = buildGeminiUsageProgress(totals.ProRequests, quota.ProRPD, resetAt, totals.ProTokens, totals.ProCost, now)
+	usage.GeminiFlashDaily = buildGeminiUsageProgress(totals.FlashRequests, quota.FlashRPD, resetAt, totals.FlashTokens, totals.FlashCost, now)
+
+	return usage, nil
+}
+
 // addWindowStats 为 usage 数据添加窗口期统计
 // 使用独立缓存（1 分钟），与 API 缓存分离
 func (s *AccountUsageService) addWindowStats(ctx context.Context, account *Account, usage *UsageInfo) {
@@ -388,3 +423,25 @@ func (s *AccountUsageService) estimateSetupTokenUsage(account *Account) *UsageIn
 	// Setup Token无法获取7d数据
 	return info
 }
+
+func buildGeminiUsageProgress(used, limit int64, resetAt time.Time, tokens int64, cost float64, now time.Time) *UsageProgress {
+	if limit <= 0 {
+		return nil
+	}
+	utilization := (float64(used) / float64(limit)) * 100
+	remainingSeconds := int(resetAt.Sub(now).Seconds())
+	if remainingSeconds < 0 {
+		remainingSeconds = 0
+	}
+	resetCopy := resetAt
+	return &UsageProgress{
+		Utilization:      utilization,
+		ResetsAt:         &resetCopy,
+		RemainingSeconds: remainingSeconds,
+		WindowStats: &WindowStats{
+			Requests: used,
+			Tokens:   tokens,
+			Cost:     cost,
+		},
+	}
+}