fix: run dummy bcrypt.compare on unknown user to prevent timing side-channel

tests/auth.test.ts

@@ -115,4 +115,20 @@ describe("verifyUserCredentials", () => {
     expect(await verifyUserCredentials(email, undefined)).toBeNull();
     expect(await verifyUserCredentials(123, password)).toBeNull();
   });
+
+  // Timing side-channel fix: an unknown email must not short-circuit to a
+  // different outcome shape than a known email with the wrong password —
+  // both must be indistinguishable (null) so a caller/attacker can't infer
+  // which case occurred from the result alone. (The actual timing-safety
+  // property — running a dummy bcrypt.compare() on the unknown-email path —
+  // isn't asserted here since timing isn't reliably testable in a unit
+  // test; this only pins down the observable behavior staying identical.)
+  it("returns the same result (null) for an unknown email as for a known email with the wrong password", async () => {
+    const unknownEmailResult = await verifyUserCredentials("[email protected]", password);
+    const wrongPasswordResult = await verifyUserCredentials(email, "wrong-password");
+
+    expect(unknownEmailResult).toBeNull();
+    expect(wrongPasswordResult).toBeNull();
+    expect(unknownEmailResult).toBe(wrongPasswordResult);
+  });
 });