feat(notifications): ntfy/Gotify push channels with live delivery (#69)

Per-user notification_channels (ntfy + Gotify): every
insertUserNotification now fans out best-effort to enabled channels —
deployment/backup/security/agent alerts reach the user's phone via the
ntfy app with zero extra infrastructure.

- POST/GET/DELETE /notifications/channels + POST .../:id/test which
  pushes a real probe so misconfiguration surfaces before alerts depend
  on it (502 DELIVERY_FAILED on endpoint errors).
- ntfy: POST to endpoint (topic in path), Title/Tags headers, optional
  Bearer token. Gotify: POST {endpoint}/message?token=...
- Delivery is best-effort by design — a dead relay never breaks the
  producing flow; one shared 5s-timeout client.
- CLI `notifications channels list|add|test|remove`, MCP tools,
  OpenAPI paths + NotificationChannel schema.
- Web settings "Push notifications" section and a Push channels card on
  the mobile notifications tab (add/test/remove).

Verified live: channel CRUD, test-ping hit a recording endpoint with
correct headers/body, and a real deployment fanned out "Deployment
succeeded" through the channel end-to-end.

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

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

app/backend/internal/mcpserver/server.go

@@ -145,6 +145,10 @@ func init() {
 	// Notifications
 	register(spec{"containr_notifications_list", "List notifications", "GET", "/notifications", nil, nil, false})
 	register(spec{"containr_notifications_read", "Mark a notification read, or all notifications with id=all", "POST", "/notifications/{id}/read", []string{"id"}, nil, false})
+	register(spec{"containr_notification_channels_list", "List push notification channels (ntfy/Gotify)", "GET", "/notifications/channels", nil, nil, false})
+	register(spec{"containr_notification_channels_create", "Add a push notification channel. Args: kind (ntfy|gotify), endpoint (full URL incl. ntfy topic or Gotify base), token (ntfy access token or Gotify app token).", "POST", "/notifications/channels", nil, []string{"kind", "endpoint", "token"}, false})
+	register(spec{"containr_notification_channels_test", "Send a test notification through a push channel. Args: id.", "POST", "/notifications/channels/{id}/test", []string{"id"}, nil, false})
+	register(spec{"containr_notification_channels_delete", "Delete a push notification channel. Args: id. Requires confirm=true.", "DELETE", "/notifications/channels/{id}", []string{"id"}, nil, true})
 
 	register(spec{"containr_webhooks_list", "List outbound webhooks", "GET", "/webhooks", nil, nil, false})
 	register(spec{"containr_webhooks_create", "Create an outbound webhook. Args: name, url, events (list, e.g. [\"service.*\",\"*\"]), secret (auto-generated if omitted), headers, enabled.", "POST", "/webhooks", nil, []string{"name", "url", "events", "secret", "headers", "enabled"}, false})