backend v2.1: 公告管理功能 + 系统重构

- 新增 Announcement 数据模型，支持公告的增删改查
- 后台管理新增"公告管理"Tab（创建/编辑/删除/启用禁用）
- 客户端 /api/announcement 改为从数据库读取
- 账号服务重构，新增无感换号、自动分析等功能
- 新增后台任务调度器、数据库迁移脚本
- Schema/Service/Config 全面升级至 v2.1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/__init__.py

@@ -1,4 +1,6 @@
-from app.services.account_service import AccountService, KeyService, LogService, GlobalSettingsService, BatchService
+from app.services.account_service import (
+    AccountService, KeyService, LogService, GlobalSettingsService, BatchService, generate_key
+)
 from app.services.auth_service import authenticate_admin, create_access_token, get_current_user
 from app.services.cursor_usage_service import (
     CursorUsageService,
@@ -7,5 +9,9 @@ from app.services.cursor_usage_service import (
     check_account_valid,
     get_account_usage,
     batch_check_accounts,
-    check_and_classify_account
+    check_and_classify_account,
+    analyze_account_from_token,
+    quick_validate_token,
+    map_membership_to_account_type,
+    calculate_usage_percent
 )