实现 Master-Slave 架构的跨区域用户和配额同步功能，支持国内主节点与
海外从节点之间的用户创建推送、余额同步、预扣费批量回传。

主要变更：
- 新增 region_sync 服务层（SyncManager/SyncClient/推送逻辑）
- User 模型扩展 Source/RemoteUserId/SyncedQuota/LastSyncAt 字段
- BillingSession 支持 SyncedUserFunding 资金来源
- 新增 SyncAuth 中间件（X-Sync-API-Key + 常量时间比较）
- 新增 PendingSyncRecord/QuotaSyncLog 数据模型
- 前端新增区域同步配置面板
- 支持 SESSION_NAME 环境变量避免多节点 cookie 冲突

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

1. Extract parseTokenKey helper function to eliminate code duplication
2. Add channelId validation to prevent invalid formats like sk-abc:def:ghi
3. Add database migration to drop deprecated bound_channel_id column

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

Add the new pricing models to both migrateDB() and migrateDBFast()
functions to enable automatic table creation on database initialization.

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