Fix #944 — snooze hint in email and Teams/Slack payloads

[erikdarlingdata]

Summary

Adds a one-line discoverability footer pointing recipients at Settings → Manage Mute Rules when an alert lands in email, Teams, or Slack — none of which can round-trip a click back to the desktop process.

Footer text:

To silence this alert: open Performance Monitor Lite → Settings → Manage Mute Rules

Files

• Lite/Services/EmailTemplateBuilder.cs — adds the hint to the HTML footer (under the existing cooldown line) and the plain-text body tail. Suppressed on the test email path.
• Lite/Services/WebhookAlertService.cs — adds the hint as a small section in the Teams MessageCard (after the facts) and as a second context-block element at the bottom of the Slack message. Suppressed on the test webhook payloads so the verification card stays minimal.

Why these placements

• Email footer/tail — already where the cooldown caveat lives, lowest-noise location.
• Teams — MessageCard sections render top-to-bottom; a trailing { text } section reads as a footnote.
• Slack — context blocks are rendered as small subtle text at the bottom of the message, which is exactly the right visual weight for a hint.

Pairs with the snooze popup from #946 to close out #944.

Test plan

• Trigger a real alert (email + Teams + Slack all configured).
• Email: verify the hint shows in both HTML and plain-text views, on a single line below the cooldown text.
• Teams: verify the hint shows below the alert facts in the message card.
• Slack: verify the hint shows in the small context line at the bottom of the message.
• Send a test email from Settings → confirm the hint is NOT shown on the test card.
• Send a test Teams + test Slack notification → confirm the hint is NOT shown on the test cards.

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