Migrate from AES-256-CBC to AES-256-GCM for authenticated encryption

- Replace CBC+HMAC with GCM (built-in authentication via authTag)
- Remove createHmac and timingSafeEqual imports (no longer needed)
- Remove Snyk-ignore comments (no longer suppressible findings)
- Update test: tampered MAC test -> tampered auth tag test
- Add one-time migration script for existing CBC-encrypted 2FA secrets

src/lib/auth/laravel-encrypter.test.ts

@@ -28,13 +28,13 @@ describe("LaravelEncrypter", () => {
     expect(enc.decryptString(payload)).toBe("hello world");
   });
 
-  it("fails closed when the MAC is tampered", () => {
+  it("fails closed when the auth tag is tampered", () => {
     const enc = new LaravelEncrypter(APP_KEY);
     const payload = enc.encrypt("x");
     const json = JSON.parse(Buffer.from(payload, "base64").toString("utf8"));
-    json.mac = "00".repeat(32);
+    json.tag = Buffer.alloc(16).toString("base64"); // zeroed auth tag
     const tampered = Buffer.from(JSON.stringify(json), "utf8").toString("base64");
-    expect(() => enc.decrypt(tampered)).toThrow(/MAC is invalid/);
+    expect(() => enc.decrypt(tampered)).toThrow();
   });
 
   it("decrypts a payload produced with a fresh instance of the same key", () => {