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/domain_constants.go

@@ -91,6 +91,9 @@ const (
 
 	// 管理员 API Key
 	SettingKeyAdminApiKey = "admin_api_key" // 全局管理员 API Key（用于外部系统集成）
+
+	// Gemini 配额策略（JSON）
+	SettingKeyGeminiQuotaPolicy = "gemini_quota_policy"
 )
 
 // Admin API Key prefix (distinct from user "sk-" keys)