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/config.rs

@@ -256,6 +256,9 @@ pub struct ServerConfig {
     pub max_upload_mb: u64,
     /// Where to look for new releases (Gitea/GitHub "latest release" API); empty = no check.
     pub update_url: String,
+    /// Bearer token for GET /metrics (Prometheus); empty = /metrics disabled.
+    #[serde(skip_serializing_if = "String::is_empty")]
+    pub metrics_token: String,
 }
 
 impl Default for ServerConfig {
@@ -275,6 +278,7 @@ impl Default for ServerConfig {
             budget_action: "warn".into(),
             max_upload_mb: 500,
             update_url: "https://git.familie-zink.org/api/v1/repos/zink/codecraft/releases/latest".into(),
+            metrics_token: String::new(),
         }
     }
 }