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/sqlc/queries/notifications.sql

@@ -17,3 +17,23 @@ UPDATE notifications SET read_at = $3 WHERE id = $1 AND user_id = $2 AND read_at
 
 -- name: MarkAllNotificationsReadByUser :exec
 UPDATE notifications SET read_at = $2 WHERE user_id = $1 AND read_at IS NULL;
+
+-- name: ListNotificationChannelsByUser :many
+SELECT id, user_id, kind, endpoint, token, enabled, created_at, updated_at
+FROM notification_channels
+WHERE user_id = $1 AND enabled
+ORDER BY created_at;
+
+-- name: ListAllNotificationChannelsByUser :many
+SELECT id, user_id, kind, endpoint, token, enabled, created_at, updated_at
+FROM notification_channels
+WHERE user_id = $1
+ORDER BY created_at;
+
+-- name: InsertNotificationChannel :one
+INSERT INTO notification_channels (user_id, kind, endpoint, token)
+VALUES ($1, $2, $3, $4)
+RETURNING id, user_id, kind, endpoint, token, enabled, created_at, updated_at;
+
+-- name: DeleteNotificationChannel :execrows
+DELETE FROM notification_channels WHERE id = $1 AND user_id = $2;