(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/config_checks.py

@@ -17,7 +17,7 @@ CRITICAL_ROOT_EXCLUDES = ("/proc/***", "/sys/***", "/dev/***", "/run/***", "/tmp
 def collect_global_config_checks(global_config: GlobalConfig) -> list[SelfCheck]:
     checks = [
         _absolute_path_check("Global backup root", global_config.backup_root),
-        _absolute_path_check("Global pobsync home", global_config.pobsync_home),
+        _absolute_path_check("Runtime state root", settings.POBSYNC_HOME),
         _runtime_backup_root_check(global_config),
         _rsync_binary_check(global_config.rsync_binary),
         _rsync_recursion_check(
@@ -97,7 +97,7 @@ def _runtime_backup_root_check(global_config: GlobalConfig) -> SelfCheck:
     return SelfCheck(
         "Runtime backup root",
         "warning",
-        "Database backup root differs from runtime POBSYNC_BACKUP_ROOT.",
+        "Database backup root differs from the runtime backup root.",
         f"database={global_config.backup_root} runtime={settings.POBSYNC_BACKUP_ROOT}",
     )