fix: address CodeRabbit review — stream tee, cache guard, base64, layout fallback

- BoardLayoutFallback: use HomarrLogoWithTitle (no board context required)
- integration-data-cache: guard cache.set with placeholder identity to
  prevent stale write-back after invalidation + test
- next-cache-handler: memoize the promise (not resolved handler) to
  prevent concurrent Redis connections
- redis-handler: tee ReadableStream before consuming in set(), hand fresh
  branch to each get() caller
- redis-handler: base64-encode body instead of number[] (3-4x smaller)

packages/api/src/integration-data-cache.ts

@@ -56,8 +56,10 @@ export const getCachedIntegrationData = async <T>(
 
   const pending = fetcher()
     .then((data) => {
-      cache.set(key, { data, timestamp: Date.now() });
-      evictOldestEntry();
+      if (cache.get(key) === placeholder) {
+        cache.set(key, { data, timestamp: Date.now() });
+        evictOldestEntry();
+      }
       return data;
     })
     .catch((error) => {