Milestone 5. One BackupManager entry point so export, import and
auto-backup share the same serializer and crypto by construction.
Explicit DTOs decouple the backup contract from the Room schema.
JCE baseline (PBKDF2-HMAC-SHA256 600k → AES-256-GCM) behind the
BackupCrypto interface with format auto-detection on decrypt; age
lands behind the same interface in milestone 6. SigV4 signer verified
against AWS's published documentation example signature. Settings in
SharedPreferences encrypted under a non-exportable AndroidKeyStore key
— replaces the now fully deprecated androidx security-crypto with the
same construction (~70 lines, zero deps); threat-model comments state
plainly that the stored auto-backup passphrase only protects against
bucket compromise. WorkManager daily periodic job, inexact by design.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
