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.
Introduce environment variables to globally and individually control background jobs:
- `WARPBOX_JOBS_ENABLED` to toggle all background workers.
- `WARPBOX_CLEANUP_ENABLED` to toggle the expired box cleanup job.
- `WARPBOX_THUMBNAIL_ENABLED` to toggle the thumbnail generation job.
Refactor background tasks into a dedicated `backend/libs/jobs` package, allowing jobs to be registered, scheduled, and conditionally run based on the new configuration flags. Additionally, update the default maximum upload size in `.env.example` to 16GB and document the new settings in the README.
