Allow site admin to specify email messages in settings.json

[Copilot]

All email subjects and bodies sent by this plugin were hardcoded. Admins had no way to customise copy, add branding, or translate notification messages without patching source.

Changes

• messageUtils.js (new) — shared renderTemplate(template, vars) that expands {key} placeholders; unknown keys are left unchanged
• update.js — reads pluginSettings.messages for footer, beginSubject, beginBody, endSubject, endBody
• handleMessage.js — reads pluginSettings.messages for subscribeSubject, subscribeBody, unsubscribeSubject, unsubscribeBody
• README.md — documents the messages block, all keys, defaults, and placeholder reference table

All fields are optional; omitting any key preserves the existing default. Existing deployments require no config changes.

Example

"ep_email_notifications": {
  "messages": {
    "footer": "\nManage your subscriptions at {padUrl}\n",
    "beginSubject": "[{padId}] editing started",
    "beginBody": "Your watched pad is being edited:\n  {padUrl}\n{footer}",
    "subscribeBody": "Confirm your subscription to {padId}:\n  {confirmLink}"
  }
}

Available placeholders:

Placeholder	Contexts
{padId}	all
{padUrl}	all
{footer}	beginBody, endBody
{confirmLink}	subscribeBody, unsubscribeBody