(refactor) Normalize runtime config labels

Hide the old pobsync_home field from the Django admin and replace legacy
operator-facing labels with runtime state root and backup root terminology.

Rename admin compatibility fieldsets, update self-check/config-check text,
and refresh management command help so Django/systemd stays the primary
mental model.

src/pobsync_backend/admin.py

@@ -34,7 +34,7 @@ class GlobalConfigAdmin(admin.ModelAdmin):
     list_display = ("name", "backup_root", "ssh_user", "ssh_port", "updated_at")
     readonly_fields = ("created_at", "updated_at")
     fieldsets = (
-        (None, {"fields": ("name", "backup_root", "pobsync_home")}),
+        (None, {"fields": ("name", "backup_root")}),
         ("SSH", {"fields": ("default_ssh_credential", "ssh_user", "ssh_port", "ssh_options")}),
         (
             "Rsync",
@@ -50,7 +50,7 @@ class GlobalConfigAdmin(admin.ModelAdmin):
         ),
         ("Defaults", {"fields": ("default_source_root", "default_destination_subdir", "excludes_default")}),
         ("Retention defaults", {"fields": ("retention_daily", "retention_weekly", "retention_monthly", "retention_yearly")}),
-        ("Legacy JSON", {"fields": ("data",), "classes": ("collapse",)}),
+        ("Compatibility data", {"fields": ("data",), "classes": ("collapse",)}),
         ("Timestamps", {"fields": ("created_at", "updated_at"), "classes": ("collapse",)}),
     )
 
@@ -76,7 +76,7 @@ class HostConfigAdmin(admin.ModelAdmin):
         ("Source", {"fields": ("source_root", "includes", "excludes_add", "excludes_replace")}),
         ("Rsync override", {"fields": ("rsync_extra_args",)}),
         ("Retention", {"fields": ("retention_daily", "retention_weekly", "retention_monthly", "retention_yearly")}),
-        ("Legacy JSON", {"fields": ("config",), "classes": ("collapse",)}),
+        ("Compatibility data", {"fields": ("config",), "classes": ("collapse",)}),
         ("Timestamps", {"fields": ("created_at", "updated_at"), "classes": ("collapse",)}),
     )