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.spec.ts

@@ -83,6 +83,27 @@ describe("getCachedIntegrationData", () => {
     expect(fetcher).toHaveBeenCalledTimes(1);
   });
 
+  test("invalidation during pending fetch prevents stale write-back", async () => {
+    let resolveOuter!: (v: { value: string }) => void;
+    const fetcher = vi.fn(
+      () =>
+        new Promise<{ value: string }>((resolve) => {
+          resolveOuter = resolve;
+        }),
+    );
+
+    const p1 = getCachedIntegrationData("integration-a", "widget:race", fetcher);
+
+    invalidateIntegrationDataCache("integration-a");
+    resolveOuter({ value: "stale" });
+    await p1;
+
+    const freshFetcher = vi.fn(async () => ({ value: "fresh" }));
+    const result = await getCachedIntegrationData("integration-a", "widget:race", freshFetcher);
+    expect(result).toEqual({ value: "fresh" });
+    expect(freshFetcher).toHaveBeenCalledTimes(1);
+  });
+
   test("invalidates all cache entries for an integration", async () => {
     const fetcherOne = vi.fn(async () => ({ value: "one" }));
     const fetcherTwo = vi.fn(async () => ({ value: "two" }));

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) => {