Meter a session by its provider, and let llama.cpp run over ssh

The rate-limit bar answered a question about an account, and picked the
answer by machine. One machine runs echo, the Claude CLI and a local
model side by side, so every echo session on it drew the CLI's five-hour
window: a quota that session cannot spend and could never run down. A
session now names its meter (`usageProvider`, from
`DriverKind::usage_provider`, which `usage::providers_for` reads too so
the two lists cannot disagree), and the phone matches on machine *and*
provider. Nothing meters echo or llama, and nothing at all is drawn --
including while the first fetch is out, since "checking" under a session
that turns out to meter nothing is a row the screen then withdraws.

Echo gets a meter it can be *told* about instead: `/usage 42`,
`/usage 95 20`, `/usage 42 never`, `/usage notloggedin`,
`/usage unreachable`, `/usage failed`, `/usage off`. Those states cost
real quota to arrange, which is why none of them had been looked at.

And llama.cpp runs wherever a setup says, which was the last of phase 5.
`Transport::reserve_port` is the second half of what a transport is --
"run this" plus "reach this port" -- returning the port the server binds
there and the port that reaches it here, and `Launch::reaching` puts the
`-L` tunnel on the connection that already carries the command. Three
things that came out of building it:

- A forwarded launch gets a pty and every other one keeps `-T`. Killing
  the ssh client ends a CLI by closing the stdin it reads; llama-server
  never reads its stdin, so the same kill left it running on the far
  machine with the model loaded -- one orphan per stopped session.
- The model is looked for on the machine that will serve it, at that
  machine's own models directory, so `GET /setups/{id}/models` is what
  the spawn screen offers rather than the backend's own downloads.
- The readiness poll watches the process, not only the port: a model
  that will not load exits in a second and would otherwise have been
  reported as "gave up after 300s". The failure carries the log's tail.

Exercised end to end against this VM over ssh to itself: spawn, load,
answer, outlive a backend restart, be adopted, answer again, and stop --
with both the ssh client and the far llama-server gone afterwards. The
local path, the Claude bar and the spawn screen checked on the emulator.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/Cargo.toml

@@ -47,6 +47,12 @@ ureq = { version = "3", features = ["json"] }
 # both in the graph rustls refuses to auto-select one.
 rustls = "0.23"
 libc = "0.2.189"
+# One ISO-8601 timestamp: the reset time on the invented rate-limit window
+# an echo session's `/usage` puts up. Already in the tree behind the
+# certificate machinery, so this is a direct name for what is compiled
+# anyway rather than a new crate -- and the alternative was hand-rolling a
+# civil-from-days conversion to print one line.
+time = { version = "0.3", features = ["formatting"] }
 
 [dev-dependencies]
 tempfile = "3"