Improve ntfy notification message templates

[borg-codex-bot]

Summary

Improve ntfy notification titles and message bodies so they are more professional, concise, and useful on mobile devices.

Scope

• Use the configured ntfy display name as title prefix.
• Use a compact success template for backup_success events.
• Prefer short target information over full repository URIs for successful backup notifications.
• Include archive name when available.
• Keep warning and failure notifications more detailed with warning/error/action context.

Accepted backup success format

Borg-Backup-UI - Backup successful

Job: Flash
Result: Successful
Duration: 19 sec
Finished: 2026-07-01 08:05
Target: Storagebox / borg-backup-flash
Archive: flash-backup-2026-07-01_08-05-11

Acceptance Criteria

• ntfy backup_success messages use the compact format above where data is available.
• The title uses the configured ntfy display name.
• Long repository URIs are not prominent in successful backup notifications.
• Tests cover the message builder behavior.