feat(backups): add optional custom name for backup files

[narcisonunez]

What is this PR about?

• Add an optional customName field to backup schedules so the generated backup file can be prefixed with a user-defined name instead of only the raw ISO timestamp (closes Name custom in the BACKUP #4961).
• Sanitize the custom name (sanitizeBackupCustomName) and validate it via zod (^[a-zA-Z0-9._-]*$, max 100 chars) on both the form and the API input.
• Fix keepLatestNBackups retention pruning, which previously relied on filenames sorting lexicographically in hronological order, but this is no longer true once a custom name can be prepended. It now sorts by real S3 modification time (rclone lsf --format "tp") instead of by filename.

Checklist

Before submitting this PR, please make sure that:

• You created a dedicated branch based on the canary branch.
• You have read the suggestions in the CONTRIBUTING.md file https://github.com/Dokploy/dokploy/blob/canary/CONTRIBUTING.md#pull-request
• You have tested this PR in your local instance. If you have not tested it yet, please do so before submitting. This helps avoid wasting maintainers' time reviewing code that has not been verified by you.

Issues related (if applicable)

closes #4961

Screenshots (if applicable)

Greptile Summary

The PR adds optional sanitized custom names to backup artifacts and changes retention pruning to sort objects by modification time rather than filename.

• Adds the nullable customName backup field, migration, API validation, and dashboard controls.
• Applies custom names consistently across database, compose, and web-server backup generation.
• Updates retention pruning and tests it against mixed timestamp-only and custom-prefixed filenames.
• Improves long backup-path display in the restore interface.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

_{Reviews (2): Last reviewed commit: "[autofix.ci] apply automated fixes" | Re-trigger Greptile}

Context used:

• Knowledge Base — Backups and Schedules