Recovery notifications for Discord, Slack, Teams, PagerDuty, and other
service webhooks were sending a generic JSON payload that lacked the
required format (e.g. Discord needs `embeds`, Slack needs `blocks`),
causing resolved notifications to silently fail.
- Add `prepareResolvedWebhookData` to build template data with Level="resolved"
- Route resolved webhooks through service-specific templates with full
URL rendering, Telegram ChatID extraction, and PagerDuty routing_key
- Custom user templates take precedence over built-in service templates
- Return errors on service template failures instead of falling back to
generic payloads that endpoints would reject
- Fix PagerDuty template to send event_action="resolve" for resolved alerts
