(feature) Add staff-only Django dashboard views

Add a small template-based UI for inspecting pobsync state through Django. The
dashboard shows host, schedule, snapshot, and backup run summaries, while host
detail pages show config, schedule, recent runs, and discovered snapshots.

Keep the views read-only and staff-protected, document the new dashboard URL,
and cover the routes with focused view tests.

src/pobsync_server/urls.py

@@ -3,10 +3,12 @@ from __future__ import annotations
 from django.contrib import admin
 from django.urls import path
 
-from pobsync_backend import api
+from pobsync_backend import api, views
 
 
 urlpatterns = [
+    path("", views.dashboard, name="dashboard"),
+    path("hosts/<str:host>/", views.host_detail, name="host_detail"),
     path("api/", api.api_index),
     path("api/status/", api.status),
     path("api/hosts/", api.hosts),