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/frontend/src/lib/api-client.ts

@@ -2596,6 +2596,42 @@ export async function markAllNotificationsRead(): Promise<void> {
   await requestJson('/notifications/read-all', { method: 'POST' });
 }
 
+export type NotificationChannel = {
+  id: string;
+  kind: 'ntfy' | 'gotify';
+  endpoint: string;
+  enabled: boolean;
+  created_at: string;
+};
+
+export async function listNotificationChannels(): Promise<NotificationChannel[]> {
+  const payload = await requestJson<{ channels?: NotificationChannel[] }>('/notifications/channels');
+  return payload.channels ?? [];
+}
+
+export async function createNotificationChannel(input: {
+  kind: 'ntfy' | 'gotify';
+  endpoint: string;
+  token?: string;
+}): Promise<NotificationChannel> {
+  const payload = await requestJson<{ channel: NotificationChannel }>('/notifications/channels', {
+    method: 'POST',
+    body: JSON.stringify(input),
+  });
+  return payload.channel;
+}
+
+export async function deleteNotificationChannel(id: string): Promise<void> {
+  await requestJson(`/notifications/channels/${encodeURIComponent(id)}`, { method: 'DELETE' });
+}
+
+export async function testNotificationChannel(id: string): Promise<void> {
+  await requestJson(`/notifications/channels/${encodeURIComponent(id)}/test`, {
+    method: 'POST',
+    body: JSON.stringify({}),
+  });
+}
+
 export type PreviewEnvironmentEntity = components['schemas']['PreviewEnvironment'];
 
 export async function listPreviewEnvironments(projectId: string): Promise<PreviewEnvironmentEntity[]> {