(feature) Add schedule editing view for hosts

Add a staff-only Django form for creating and updating host schedules using the
SQL-backed ScheduleConfig model. Link the form from host detail pages, validate
cron expressions with the existing scheduler parser, and preserve scheduler/CLI
behavior by writing to the same source of truth.

Cover default rendering, schedule creation, updates, and invalid cron handling
with view tests.

src/pobsync_server/urls.py

@@ -11,6 +11,7 @@ urlpatterns = [
     path("hosts/<str:host>/", views.host_detail, name="host_detail"),
     path("hosts/<str:host>/discover-snapshots/", views.discover_host_snapshots, name="discover_host_snapshots"),
     path("hosts/<str:host>/retention-plan/", views.host_retention_plan, name="host_retention_plan"),
+    path("hosts/<str:host>/schedule/", views.edit_host_schedule, name="edit_host_schedule"),
     path("api/", api.api_index),
     path("api/status/", api.status),
     path("api/hosts/", api.hosts),