feat: handle RSA key alignment when migrating to shared XTM Composer

[ncarenton]

Parent: #86

Context

Each XTM Composer instance has its own RSA private key (used for connector credentials encryption). The key is loaded as a singleton at startup (src/main.rs — private_key()). When migrating existing customers to a shared instance, the two former instances have different keys. Connectors encrypted with the old key won't decrypt with the new one.

Options

1. Support multiple keys (quick workaround): Allow the shared composer to hold both private keys and try decryption with each. Enables seamless migration without re-encrypting credentials on the platform side.
2. RSA key renewal/rotation (clean solution): Add the ability to rotate the RSA key on the platform side, so both platforms can be re-keyed to match the shared composer's key. Already tracked in feat: handle renewal of the RSA key #72.

Both options may be valuable — option 1 for migration, option 2 for ongoing operational needs.