Tests pass under heavy host load (closes #233)

The suite timed out when this host was heavily loaded. vitest now runs at most four workers instead of one per CPU core. `test/cli/bin.test.ts` loads the sources `bin/quak.ts` imports and starts libsodium once, before its tests, so no test pays for the first load. The fake accounts in `test/ente-server.ts` and `test/thumbnails/thumbnails.test.ts` stretch their passwords at libsodium's lowest Argon2id cost, since every login stretches them again.

Judgement call: the login, usage, unwrap and KDF tests keep the realistic 64 MiB cost.

Model: opus-5-5

test/ente-server.ts

@@ -19,8 +19,11 @@ import { SRP, SrpServer } from "fast-srp-hap";
 import { toBase64, deriveKEK, deriveLoginSubkey } from "../src/crypto/index.js";
 import type { KeyAttributes } from "../src/auth/types.js";
 
-const TEST_OPS = 2;
-const TEST_MEM = 64 * 1024 * 1024;
+// The account's password is stretched at libsodium's lowest Argon2id cost.
+// Every login stretches it again, and at a realistic cost that takes seconds on
+// a loaded host.
+const TEST_OPS = 1;
+const TEST_MEM = 8192;
 
 export interface MetaMockState {
     verifier: Buffer;