- Add resolveSecretsDeep() to recursively resolve secret placeholders
(env://, file://, encv1:) in any config field
- Refactor restic operations to resolve secrets once via resolveAndBuild()
- Refactor volume backend dispatcher to resolve secrets at mount time
- Remove redundant per-field resolveSecret() calls from SMB, WebDAV, SFTP backends
- Simplify notifications service by replacing switch-based decryption
- Update backend-compatibility to use deep resolution for credential comparison
This allows users to use secret references in any configuration field,
not just the designated sensitive fields.
* feat: add support for SFTP volume backend
* feat: allow more secure SFTP connection with known hosts
* refactor: make default value for skip host key check to false
* refactor: use os.userInfo when setting uid/gid in mount commands
