feat(database): add email notifications for backups/imports & backup verification

[Ludy87]

Description of Changes

What was changed

• Added enterprise configuration for database-related email notifications:
  • premium.enterpriseFeatures.databaseNotifications.backups.successful|failed
  • premium.enterpriseFeatures.databaseNotifications.imports.successful|failed
  • Extended ApplicationProperties and settings.yml.template accordingly.
• Introduced DatabaseNotificationServiceInterface and DatabaseNotificationService to centralize and gate notification sending (EE-only, respects mail.enabled).
• Wired notifications into DatabaseService:
  • Sends emails on successful/failed backups and imports.
  • Added backup verification step:
    • Compute SHA‑256 checksum for backup files.
    • Validate backup by loading it into an in‑memory H2 instance (RUNSCRIPT) before using it.
    • Abort import if verification fails.
• Enhanced EmailService:
  • Added sendSimpleMail(String to, String subject, String body) (async) for lightweight notifications.
  • Added debug logging for successful sends.
• Minor refactors and improved logging around backup/export/import flows.

Why the change was made

• Provide admins with timely, configurable notifications about critical database operations (backups/imports).
• Increase reliability by verifying backup integrity before execution, reducing risk from corrupted or incomplete scripts.
• Keep configuration explicit and self-documenting via new keys in settings.yml.template.

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Checklist

General

• [x] I have read the Contribution Guidelines
• [x] I have read the Stirling-PDF Developer Guide (if applicable)
• [ ] I have read the How to add new languages to Stirling-PDF (if applicable)
• [x] I have performed a self-review of my own code
• [x] My changes generate no new warnings

Documentation

• [ ] I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• [ ] I have read the section Add New Translation Tags (for new translation tags only)

UI Changes (if applicable)

• [ ] Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• [ ] I have tested my changes locally. Refer to the Testing Guide for more details.

[Frooodle]

Interesting idea.. hmmm