fix(notifications): send server threshold alerts through email, resend, gotify and ntfy

[Siumauricio]

Closes #3518

sendServerThresholdNotifications fetched the email relation but never sent anything through it — the loop only handled discord, telegram, slack, mattermost, custom, lark, pushover and teams. Users who configured an email notification with the Server Threshold trigger enabled silently got nothing. Resend, Gotify and Ntfy had the same gap.

Adds the four missing channels following the same patterns as database-backup.ts, with a new server-threshold email template.

Greptile Summary

The PR completes server-threshold notification delivery for SMTP email, Resend, Gotify, and Ntfy.

• Adds a React Email template containing server, metric, threshold, message, and timestamp details.
• Eager-loads the additional channel relations and dispatches channel-specific threshold alerts.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete changed-code failure identified.

The newly added channel relations, template properties, sender arguments, and channel-specific payloads align with the existing notification contracts and sibling dispatch implementations.

_{Reviews (1): Last reviewed commit: "fix(notifications): send server threshol..." | Re-trigger Greptile}

Context used:

• Knowledge Base — Global Settings, Docker Registries, and Notifications