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>

当模型配置了渠道定价但全局定价未设置时，不再提前报错。
将全局定价检查延迟到渠道选择后执行。

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

This PR fixes three issues:
1. Token bound_channel_id not updating: Add bound field to Update() method
2. Channel pricing quota_type detection: Use QuotaTypeByCall constant instead of magic number 1
3. Log display for ratio-based pricing: Set ModelPrice=-1 for ratio-based pricing

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

Integrate channel-specific pricing into the billing logic.
- First tries to get channel pricing using model.GetEffectivePricing
- If found, uses channel pricing (modelRatio, completionRatio, modelPrice, usePrice)
- If not found, falls back to global pricing from ratio_setting

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