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/mobile/src/api.ts

@@ -94,6 +94,14 @@ export interface AppNotification {
   created_at?: string;
 }
 
+export interface NotificationChannel {
+  id: string;
+  kind: 'ntfy' | 'gotify';
+  endpoint: string;
+  enabled: boolean;
+  created_at?: string;
+}
+
 // Masked value the API returns for secret variables; echoing it back on
 // update preserves the stored ciphertext.
 export const MASKED_SECRET = '********';
@@ -238,6 +246,33 @@ export class Api {
     return this.req<unknown>('POST', '/notifications/read-all');
   }
 
+  notificationChannels() {
+    return this.req<{ channels: NotificationChannel[] }>(
+      'GET',
+      '/notifications/channels',
+    ).then((r) => r.channels ?? []);
+  }
+
+  addNotificationChannel(input: {
+    kind: 'ntfy' | 'gotify';
+    endpoint: string;
+    token?: string;
+  }) {
+    return this.req<{ channel: NotificationChannel }>(
+      'POST',
+      '/notifications/channels',
+      input,
+    );
+  }
+
+  deleteNotificationChannel(id: string) {
+    return this.req<unknown>('DELETE', `/notifications/channels/${id}`);
+  }
+
+  testNotificationChannel(id: string) {
+    return this.req<unknown>('POST', `/notifications/channels/${id}/test`, {});
+  }
+
   serviceDeployments(id: string) {
     return this.req<{ deployments: Deployment[] }>(
       'GET',