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>

web/src/i18n/en.ts

@@ -568,6 +568,10 @@ export const en = {
   "settings.priceCacheWrite": "Cache write",
   "settings.addProvider": "Add provider",
   "settings.providerName": "Name (optional)",
+  "settings.providerUp": "reachable · {ms} ms",
+  "settings.providerDown": "unreachable",
+  "settings.providerCheck": "Check providers",
+  "settings.providerChecking": "Checking…",
   "settings.reloadProviders": "Reload providers & models",
   "settings.deleteProviderConfirm": "Delete provider \"{name}\"?",
   "settings.invalidName": "Names may only contain letters, digits, - and _.",