Refactor the admin storage backend creation and editing flows to use
provider-specific pages (e.g., `/admin/storage/new/sftp`) instead of a
single generic form. This ensures only relevant fields are rendered for
each storage provider (such as SFTP, S3, or WebDAV).
Additionally:
- Prevent mutation of the storage provider type during backend edits.
- Add comprehensive unit tests for provider-specific rendering, edit
validation, and CSRF/admin route protection.
