use decrementing amount of redirects

This makes `redirects` contain the number of remaining redirects, which
makes it easier to limit the number of further redirects that should be
allowed.

packages/backend/src/remote/activitypub/request.ts

@@ -96,7 +96,7 @@ export async function signedGet(_url: string, user: { id: User['id'] }): Promise
 	let url = _url;
 	const keypair = await getUserKeypair(user.id);
 
-	for (let redirects = 0; redirects < 3; redirects++) {
+	for (let redirects = 3; redirects > 0; redirects--) {
 		const req = createSignedGet({
 			key: {
 				privateKeyPem: keypair.privateKey,