Usage History: see which client made each request (opt-in)

[ianderse]

When one oMLX server serves several tools (editors, agents, Home Assistant), Usage History shows which model served the tokens but not who asked for them. #1912 asked for this, and #3038 explains why it isn't possible today.

Proposal: an opt-in setting, Track usage per client (off by default). It adds a Clients breakdown to Usage History, in the web dashboard and the macOS app, with three tabs: All (each key and IP pair), By key, and By IP.

• Each request records which API key authenticated it (the main key, a sub key's name, or none) and its peer IP. Nothing else: no key, no headers, no request content.
• X-Forwarded-For is not trusted, since any client can set it. Behind a reverse proxy, By IP shows the proxy's address, and distinct sub keys still tell clients apart.
• It uses a new client_usage_hourly table, created without bumping the schema version, so downgrading to an older oMLX keeps model history readable.
• Turning it off stops new attribution and keeps the rows already recorded.

I built it and opened a PR: #4207. It changes about 1,500 lines across 37 files, so I'm checking the approach with you first, as the contributing guide asks.

Questions:

• Is per-client history something you want in Usage History?
• Is a separate hourly table the right storage, or would you rather extend the existing one?
• Are you comfortable with IP-level tracking behind an opt-in switch? The docs currently say oMLX records no client IPs, and the PR amends that to say it holds unless you opt in.