DC-019: fix flaky backup-manager tamper test (authTag byte corruption)

The 'rejects tampered data (auth tag mismatch)' test corrupted the
encrypted blob by replacing its first base64 char with 'X'. When the
random 16-byte IV's first base64 char was already 'X' (~1/64 chance),
the replacement was a no-op and decryption succeeded — causing the test
to flake ~1.6% of runs.

Fix: parse the iv:authTag:ciphertext format, XOR the first authTag byte
with 0xFF (guaranteed to change the value), reassemble. This reliably
triggers the AES-256-GCM integrity failure every time.

Verified: 30/30 isolated runs + 8/8 full-suite runs (1036/1036), zero
failures. The production encryptBackup/decryptBackup (AES-256-GCM)
code is correct and unchanged.

dashcaddy-api/__tests__/backup-manager.test.js

@@ -184,9 +184,16 @@ describe('BackupManager — backup/restore lifecycle', () => {
     it('rejects tampered data (auth tag mismatch)', async () => {
       const data = Buffer.from('test');
       const encrypted = await backupManager.encryptBackup(data, testKey);
-      // Corrupt the first character of the IV
-      const str = encrypted.toString();
-      const tampered = Buffer.from('X' + str.substring(1));
+      // Corrupt the authTag so the GCM integrity check is guaranteed to fail.
+      // The format is iv:authTag:ciphertext (all base64). We flip all bits of
+      // the first authTag byte — XOR with 0xFF always changes the value, so
+      // this can never be a no-op (unlike replacing a base64 char with a fixed
+      // char, which collides ~1/64 of the time when that char already matches).
+      const parts = encrypted.toString().split(':');
+      const authTagBuf = Buffer.from(parts[1], 'base64');
+      authTagBuf[0] ^= 0xFF;
+      parts[1] = authTagBuf.toString('base64');
+      const tampered = Buffer.from(parts.join(':'));
       await expect(backupManager.decryptBackup(tampered, testKey))
         .rejects.toThrow();
     });