feat(BA-1156): Customize SMTP template

[jopemachine]

resolves #4168 (BA-1156).

Checklist: (if applicable)

• Mention to the original issue

[Copilot]

Pull Request Overview

This PR adds support for customizable SMTP email templates for the Backend.AI SMTP Reporter to resolve BA-1156. Key changes include adding a new "template" parameter to the SMTP configuration, updating the SMTPReporter to generate email bodies using the provided template, and modifying both the default configuration and sample TOML file with the new template and trigger-policy changes.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/ai/backend/manager/server.py	Added "template" argument when initializing the SMTPReporter configuration.
src/ai/backend/manager/reporters/smtp.py	Updated SMTPReporter to build email bodies from a configurable template.
src/ai/backend/manager/config.py	Introduced the default SMTP template and updated the config schema.
configs/manager/sample.toml	Added the template setting and updated the trigger-policy.
changes/4207.feature.md	Documented the new functionality for customizing SMTP templates.