[FEATURE] Track and limit user's token usage

[takasaki404]

We should track and limit a user's token usage per day. Refresh on the next day.

[takasaki404]

1. Track and limit token usage at backend.
2. Create a new endpoint (for example, GET /user/token-limit -> { "total": 10000, "used": 1000, "next_refresh_at": "2025-07-18T09:00:00"})

[Wes1eyyy]

Will do

[Wes1eyyy]

Sub-enhancement: Refactor User Service in order to manage WalletLogin and EmailLogin features

[takasaki404]

Sub-enhancement: Refactor User Service in order to manage WalletLogin and EmailLogin features

没必要，privy的user id可以直接当成wallet address, 用现有的数据库layout就能存储。用户绑定的email / external wallet address 可以通过privy获取到。

[takasaki404]

主要是前端直接通过privy获取钱包地址没有稳定接口，privy也支持一个账户连接很多钱包，以及email login 默认创建embedding钱包之类的，没必要增加复杂性。