add built-in OAuth account routing and quota management

[djgilcrease]

Problem
T3 can display subscription usage from CLIProxyAPI, but sending agent requests through an account pool still requires operating and configuring a separate service. Users need one T3 environment to own that pool, other connected environments to use it, and quota visibility alongside ordinary Usage limits.

I opened and then closed #16677 because it is a large feature that should be discussed and approved by the core devs.