[Enhancement]: Put the backup status back in the email subject #209

Current Behavior

Since v4.0.0 (commit 1ef4038, the redesigned notification emails), the backup notification title, which is also the email subject, no longer starts with the outcome of the run:

Source: src/lib/notifications/templates/backup.ts. renderNotificationEmail uses payload.title as the subject.

A successful run now only says "finished", which does not say whether the backup succeeded. A partial run also starts with "finished", so in an inbox it looks almost the same as a success.

Proposed Improvement

Put the status back at the start of the subject, as it was before v4.0.0:

• Backup Successful: <job>
• Backup Partial: <job> (or Backup Warning: <job>)
• Backup Failed: <job>

The new email body, preheader and layout can stay as they are. Only the subject (title) needs the status word.

Motivation

Putting the status in the subject of a backup or monitoring alert is a common convention. Many teams depend on it:

• You can see the result in the inbox list without opening the mail.
• Mail rules, filters, ticketing systems and alert pipelines match on words like "Successful" or "Failed". Those rules stopped working after the upgrade to v4.
• A partial run is a warning that needs attention, so it must not look like a success.

Area

Notifications

Additional Context

Related: #46 (the job name was added to the subject). This keeps the job name and adds the status back in front of it.