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/de.ts

@@ -569,6 +569,10 @@ export const de: Messages = {
   "settings.priceCacheWrite": "Cache schreiben",
   "settings.addProvider": "Provider hinzufügen",
   "settings.providerName": "Name (optional)",
+  "settings.providerUp": "erreichbar · {ms} ms",
+  "settings.providerDown": "nicht erreichbar",
+  "settings.providerCheck": "Provider prüfen",
+  "settings.providerChecking": "Prüfe…",
   "settings.reloadProviders": "Provider & Modelle neu laden",
   "settings.deleteProviderConfirm": "Provider „{name}“ löschen?",
   "settings.invalidName": "Namen dürfen nur Buchstaben, Ziffern, - und _ enthalten.",