Fix activity verification after key rotation

What does this PR do?

Fixes two minor bugs in the key re-fetch logic:

• After a user's keys rotate, the very next activity will fail to validate. All further ones succeed.
• If a user's keys are deleted for some reason, they will stick around in the cache for up to 12 hours.

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packages/backend/src/core/activitypub/ApDbResolverService.ts

@@ -176,9 +176,7 @@ export class ApDbResolverService implements OnApplicationShutdown {
 	public async refetchPublicKeyForApId(user: MiRemoteUser): Promise<MiUserPublickey | null> {
 		await this.apPersonService.updatePerson(user.uri);
 		const key = await this.userPublickeysRepository.findOneBy({ userId: user.id });
-		if (key != null) {
-			await this.publicKeyByUserIdCache.set(user.id, key);
-		}
+		this.publicKeyByUserIdCache.set(user.id, key);
 		return key;
 	}