fix(19): WR-03 make scrypt hashing async (threadpool) to avoid event-loop starvation DoS

apps/api/src/routes/localAuth.ts

@@ -99,8 +99,10 @@ export function resetLoginAttempts(username: string): void {
 
 // Pre-computed dummy hash used to run verifyPassword on unknown-username paths
 // (timing defense — prevents timing-oracle username enumeration, RESEARCH Pitfall 2).
-// Computed once at module load time; the actual value is never used for auth.
-const DUMMY_HASH = hashPassword('dummy-constant-time-filler-xyzzy');
+// WR-03: hashPassword is now async. Kick off the computation once at module load and keep
+// the PROMISE; the login handler awaits it. The value is never used for auth — only to make
+// the unknown-username path perform the same scrypt work as the known-username path.
+const dummyHashPromise: Promise<string> = hashPassword('dummy-constant-time-filler-xyzzy');
 
 // ---------------------------------------------------------------------------
 // POST /local/login  →  POST /api/auth/local/login
@@ -160,9 +162,10 @@ localAuthRouter.post('/local/login', zValidator('json', loginSchema, noEchoHook)
   // ALWAYS run verifyPassword — even for unknown usernames — to prevent timing-oracle
   // username enumeration (T-19-12 / RESEARCH Pitfall 2). Use a pre-computed dummy hash
   // so the scrypt work is always performed regardless of whether username was found.
+  // WR-03: verifyPassword is async (threadpool scrypt) — await it.
   const valid = cred
-    ? verifyPassword(cred.passwordHash, password)
-    : verifyPassword(DUMMY_HASH, password);
+    ? await verifyPassword(cred.passwordHash, password)
+    : await verifyPassword(await dummyHashPromise, password);
 
   if (!valid || !cred) {
     // Increment failure counter (keyed on username)