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/main.rs

@@ -10,12 +10,14 @@ mod docs;
 mod export;
 mod files;
 mod git;
+mod health;
 mod gitcreds;
 mod hooks;
 mod images;
 mod jobs;
 mod lsp;
 mod mentions;
+mod metrics;
 mod mcp;
 mod permission;
 mod prompts;
@@ -36,9 +38,13 @@ use tracing_subscriber::EnvFilter;
 
 #[tokio::main]
 async fn main() -> Result<()> {
-    tracing_subscriber::fmt()
-        .with_env_filter(EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("info,codecraft=info")))
-        .init();
+    // CODECRAFT_LOG_FORMAT=json: one JSON object per line (Loki, Elastic, …).
+    let filter = EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("info,codecraft=info"));
+    if std::env::var("CODECRAFT_LOG_FORMAT").is_ok_and(|f| f.eq_ignore_ascii_case("json")) {
+        tracing_subscriber::fmt().json().flatten_event(true).with_current_span(false).with_env_filter(filter).init();
+    } else {
+        tracing_subscriber::fmt().with_env_filter(filter).init();
+    }
 
     // In Docker, HOME lives on the data volume; make sure it exists for git/npm.
     if let Ok(home) = std::env::var("HOME") {
@@ -57,6 +63,7 @@ async fn main() -> Result<()> {
     terminal::spawn_reaper(app.clone());
     jobs::spawn_scheduler(app.clone());
     update::spawn_checker(app.clone());
+    health::spawn_checker(app.clone());
     let router = api::router(app);
     let listener = tokio::net::TcpListener::bind(&bind).await?;
     tracing::info!("listening on http://{bind}");