Merge upstream/main: v0.1.62-v0.1.64 updates

Major updates:
- fix(gateway): fix SSE streaming usage data overwrite issue
- feat(purchase): add purchase subscription iframe page and config
- fix(scheduler): fix race condition from empty cache
- feat(settings): add PurchaseSubscriptionEnabled and PurchaseSubscriptionURL

Resolved conflicts:
- backend/cmd/server/VERSION: updated to 0.1.61
- setting_service.go: keep TianShuAPI site name while adding purchase subscription features

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/api/admin/settings.ts

@@ -28,6 +28,8 @@ export interface SystemSettings {
   doc_url: string
   home_content: string
   hide_ccs_import_button: boolean
+  purchase_subscription_enabled: boolean
+  purchase_subscription_url: string
   // SMTP settings
   smtp_host: string
   smtp_port: number
@@ -81,6 +83,8 @@ export interface UpdateSettingsRequest {
   doc_url?: string
   home_content?: string
   hide_ccs_import_button?: boolean
+  purchase_subscription_enabled?: boolean
+  purchase_subscription_url?: string
   smtp_host?: string
   smtp_port?: number
   smtp_username?: string