perf: fix caching (#8660)

The cache implementation did previously not store the results of the
computation and was thus not a cache at all. This can cause a significant
number of database queries each time someone with a large number of
followers does something that causes an activity to be federated.

packages/backend/src/misc/cache.ts

@@ -48,6 +48,7 @@ export class Cache<T> {
 
 		// Cache MISS
 		const value = await fetcher();
+		this.set(key, value);
 		return value;
 	}