Add Prometheus metrics, JSON logs and provider health checks

- GET /metrics (bearer server.metrics_token): build info, users, sessions,
  pending requests, per-model request/token/cost counters, provider_up
- CODECRAFT_LOG_FORMAT=json for one JSON object per log line
- Providers are probed every 5 minutes via their model list; status shown
  in the provider settings with a manual re-check

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/app.rs

@@ -79,6 +79,8 @@ pub struct App {
     pub terminals: crate::terminal::Terminals,
     /// Newer release found by the update check (admins see it).
     pub update: crate::update::UpdateState,
+    /// Provider reachability (checked every 5 minutes).
+    pub health: crate::health::Health,
 }
 
 /// What a user may do with a session.
@@ -118,6 +120,7 @@ impl App {
             lsp: Default::default(),
             push: crate::push::Push::new(&data),
             terminals: Default::default(),
+            health: Default::default(),
             update: Default::default(),
         });
         crate::auth::bootstrap(&app)?;