Allow drag-and-drop reordering of backup schedules

[Rajdave69]

Adds drag-and-drop functionality to reorder backup schedules

Introduces a sort order field to the database schema and implements API endpoints to update the schedule order.

This allows users to visually prioritize their backup jobs.

Partially finishes #131. This PR only addresses backup-schedules though

Summary by CodeRabbit

• New Features

  • Drag-and-drop reordering of backup schedules with updated compact backup cards (grip, status, volume/repo, Schedule / Last / Next).

• Chores

  • Persisted order: reorder actions are saved server-side so custom schedule order is retained.
  • Added drag-and-drop support libraries to enable the new behavior.

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