API: add GET endpoints for volume backup schedules and executions

[dementiev27]

Volume backups (#10946) can be created, replaced, run and deleted through the API, but not read. Database backups can. This makes it impossible to check from the API whether a volume is backed up, or whether its last run reached S3.

Current state (v4.6.0, same on main)

routes/api.php and openapi.json define these routes for applications, databases and services:

• PUT /{type}/{uuid}/storages/{storage_uuid}/backups
• POST /{type}/{uuid}/storages/{storage_uuid}/backups/run
• DELETE /{type}/{uuid}/storages/{storage_uuid}/backups

There is no GET. A GET on the same path returns:
404 {"message":"Not found.","docs":"[https://coolify.io/docs](https://coolify.io/docs)"}

This is the same response as for a missing resource, so a client reads it as "this volume has no backup schedule". I hit this on a volume that does have a schedule (visible in the UI under the application's Backups page).

For comparison, database dump backups have:

• GET /databases/{uuid}/backups
• GET /databases/{uuid}/backups/{scheduled_backup_uuid}/executions

Proposal

Mirror the database backup read endpoints for volumes:

• GET /{type}/{uuid}/storages/{storage_uuid}/backups
  Returns the existing VolumeBackupScheduleResponse (the same body PUT already returns). 404 only when the storage has no schedule.
• GET /{type}/{uuid}/storages/{storage_uuid}/backups/executions
  Returns recent runs from scheduled_volume_backup_executions: uuid, status, message, size, filename, s3_uploaded, created_at, finished_at.