Feature: add customizable email subjects bodies

[BuddiesD]

Summary

Adds customizable email subject and body templates for alert emails in TacticalRMM.

Changes

• adds configurable subject/body template fields to core settings
• supports dedicated templates for:
  • checks
  • tasks
  • agent offline
  • agent recovered
• adds section-aware template rendering in the backend
• exposes existing alert context values as template variables, including:
  • {agent}
  • {client}
  • {site}
  • {policy}
  • {alert_name}
  • section-specific variables such as {check_name} and {task_name}
• adds {site_id} as an additional template variable
• keeps {subject} and {body} available as the original generated values
• adds and updates backend tests for template rendering and settings persistence

Notes

pr-trmm.mp4

Related

Frontend PR:

• Add customizable email template UI tacticalrmm-web#54

Docs PR:

• Document customizable email templates trmm-docs#340

[BuddiesD]

I added last_response and a time format migration from Quasar to Python
If that's not wanted, I can remove them from the pull request