fix(beszel): harden cache layer and prevent blank widgets

- Add debug logging across Beszel integration, request handler, and tRPC
  router (auth, fetch, SSE, per-procedure entry/exit) gated behind
  LOG_LEVEL=debug
- cached-request-handler: retry with backoff, isValid predicate to skip
  caching invalid data, in-flight dedup (single-flight per channel) to
  avoid thundering herd after cache invalidation
- redis channel: set-then-publish so storage is always consistent even
  when pubsub publish fails
- Beszel handlers: opt into fallbackToStaleOnError and retry so brief
  upstream hiccups don't throw away good cached data
- beszel-system-stats: add refetchInterval matching each time period's
  cache TTL so historical charts refresh in the background
- tRPC router: switch Promise.all -> Promise.allSettled for getSystems
  and getAlerts; getSystemStats returns an error object instead of
  throwing. Each result carries an optional error field so one failed
  integration no longer nukes the whole widget
- Widgets: add explicit isPending loader, empty state for grid and
  stats, and a shared per-integration error indicator (tooltip with
  failed integration names). Alerts and table now only show their
  empty state after loading completes
- Translation: add empty.noSystems key to beszelSystemGrid and
  beszelSystemStats

Addresses #6030

packages/request-handler/src/lib/cached-integration-request-handler.ts

@@ -18,6 +18,9 @@ interface Options<TData, TKind extends IntegrationKind, TInput extends Record<st
   requestAsync: (integration: IntegrationOfKind<TKind>, input: TInput) => Promise<TData>;
   cacheDuration: Duration;
   cacheDurationForInput?: (input: TInput) => Duration | undefined;
+  fallbackToStaleOnError?: boolean;
+  retry?: { attempts?: number; delayMs?: number };
+  isValid?: (data: TData) => boolean;
 }
 
 export const createCachedIntegrationRequestHandler = <
@@ -35,6 +38,9 @@ export const createCachedIntegrationRequestHandler = <
           return await options.requestAsync(input.integration, input.options);
         },
         cacheDuration: options.cacheDurationForInput?.(itemOptions) ?? options.cacheDuration,
+        fallbackToStaleOnError: options.fallbackToStaleOnError,
+        retry: options.retry,
+        isValid: options.isValid,
         createRedisChannel(input, handlerOptions) {
           return createIntegrationOptionsChannel<TData>(input.integration.id, handlerOptions.queryKey, input.options);
         },