feat: webhook alerts for monitor + domain events (merge plan M2)

Generic webhook notification delivery configured via the
notify_webhook settings key — no new schema. Fires on monitor
up/down transitions (pending stays silent), domain expiry,
domain expiring, and TLS cert expiring; dispatch is async and
failures are logged, never fatal.

POST /api/notify/test verifies configuration; Settings dialog
gains a webhook URL field with save + send-test. Payload carries
event/name/text/content/message so Slack, Discord, ntfy, and
custom receivers all work.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/api/api.go

@@ -90,6 +90,7 @@ func (s *Server) routes() *gin.Engine {
 	v1.DELETE("/domains/:id", s.deleteDomain)
 	v1.POST("/domains/:id/refresh", s.refreshDomainH)
 	v1.GET("/domains/:id/checks", s.domainChecks)
+	v1.POST("/notify/test", s.notifyTest)
 
 	v1.GET("/status", s.getStatus)
 	v1.GET("/widgets/types", s.widgetTypes)