review classifier: model, effort and review concurrency as swarm-wide settings

The classifier ran every classification on a hard-coded `haiku` with no
`--effort`, and the controller ran at most two requested reviews at once,
also hard-coded, so changing any of them took a code change and a
redeploy.

swarm-controller now keeps one settings document,
`{model, effort, max_concurrent_reviews}`, in its own
`review-classifier-settings` KV bucket under `all`, the way it keeps the
schedule definitions. No document means today's values: haiku, no
`--effort`, two reviews. `GET /api/review-classifier/settings` reads it;
`PUT` replaces it whole, needs the gateway's `Remote-User` like the
try-out POST, and refuses a bad model (the inference route's check), an
effort outside the five levels or a limit outside 1..=8 with
problem+json 400.

Every run request (try-out and requested review) now carries the model
and effort, and the classifier passes them to claude instead of its
constant, which is gone. A try-out reads the settings when it is posted;
a review reads them for the limit before it waits for a place and again
for the model and effort once it has one. The review semaphore is
replaced by a running count under a resizable limit, so a PUT, or the
next review's read, changes how many run without a restart, and a lower
limit lets the runs already going finish. MAX_WAITING_REVIEWS is
unchanged.

Closes #5151, part of #5096.

swarm-controller/src/inference.rs

@@ -61,7 +61,7 @@ impl From<AgentInference> for InferenceSettings {
 
 /// A model name as the agent passes it to claude: one non-empty word that
 /// cannot be read as a command-line flag.
-fn check_model(name: &str, what: &str) -> Result<(), String> {
+pub(crate) fn check_model(name: &str, what: &str) -> Result<(), String> {
     if name.is_empty() {
         return Err(format!("the {what} is empty"));
     }