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/api/routes.go

@@ -364,6 +364,10 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 			authed.GET("/notifications", handleListNotifications)
 			authed.POST("/notifications/:id/read", handleMarkNotificationRead)
 			authed.POST("/notifications/read-all", handleMarkAllNotificationsRead)
+			authed.GET("/notifications/channels", handleListNotificationChannels)
+			authed.POST("/notifications/channels", handleCreateNotificationChannel)
+			authed.POST("/notifications/channels/:id/test", handleTestNotificationChannel)
+			authed.DELETE("/notifications/channels/:id", handleDeleteNotificationChannel)
 
 			// WebSocket endpoint
 			authed.GET("/ws", handleWebSocket)