(feature) Add host config editing view

Add a staff-only Django form for editing operational host settings while keeping
host identity stable. Support address, enablement, SSH/source overrides,
include/exclude lists, rsync extra args, and retention settings using the same
SQL-backed HostConfig model consumed by backup and scheduler flows.

Parse newline-separated list fields into JSON lists, preserve nullable
excludes_replace semantics, and cover rendering plus update behavior with view
tests.

src/pobsync_server/urls.py

@@ -9,6 +9,7 @@ from pobsync_backend import api, views
 urlpatterns = [
     path("", views.dashboard, name="dashboard"),
     path("hosts/<str:host>/", views.host_detail, name="host_detail"),
+    path("hosts/<str:host>/config/", views.edit_host_config, name="edit_host_config"),
     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"),