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/migrations_goose/20261116000000_notification_channels.sql

@@ -0,0 +1,16 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS public.notification_channels (
+    id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id uuid NOT NULL REFERENCES public.users(id) ON DELETE CASCADE,
+    kind varchar(20) NOT NULL CHECK (kind IN ('ntfy', 'gotify')),
+    endpoint text NOT NULL,
+    token varchar(255),
+    enabled boolean NOT NULL DEFAULT true,
+    created_at timestamptz DEFAULT now(),
+    updated_at timestamptz DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_notification_channels_user ON public.notification_channels (user_id);
+
+-- +goose Down
+DROP TABLE IF EXISTS public.notification_channels;