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>

ROADMAP2.md

@@ -508,8 +508,10 @@ and wired refs, one click or one CLI call.
   - [x] **env var edit + domain management + replica scaling** (PR #61 —
     mobile v1.5: variables editor, domains screen, scale + sleep
     settings, notifications tab). **cron execution history** still open.
-  - [ ] **Push notifications** — zero-infra path = self-hosted
-    ntfy/Gotify relay (no Firebase dependency), FCM/APNs optional.
+  - [x] **Push notifications** — per-user `notification_channels`
+    (ntfy + Gotify): every `insertUserNotification` fans out best-effort
+    to enabled channels; CRUD + live test-ping via web settings, CLI
+    `notifications channels`, and MCP. FCM/APNs remain optional.
   - [ ] **Offline resilience** — cached last-known state + unreachable
     banner.
 - [ ] **Status/offline resilience** — app + CLI degrade gracefully when