(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/self_check.py

@@ -76,10 +76,17 @@ def _django_checks() -> list[SelfCheck]:
 
 def _path_checks() -> list[SelfCheck]:
     checks = []
-    checks.append(_path_check("POBSYNC_HOME", Path(settings.POBSYNC_HOME), must_be_absolute=True, must_be_writable=True))
     checks.append(
         _path_check(
-            "POBSYNC_BACKUP_ROOT",
+            "State root",
+            Path(settings.POBSYNC_HOME),
+            must_be_absolute=True,
+            must_be_writable=True,
+        )
+    )
+    checks.append(
+        _path_check(
+            "Backup root",
             Path(settings.POBSYNC_BACKUP_ROOT),
             must_be_absolute=True,
             must_exist=True,
@@ -259,7 +266,7 @@ def _config_checks() -> list[SelfCheck]:
     message = "Default global config exists."
     if global_config.backup_root != settings.POBSYNC_BACKUP_ROOT:
         status = "warning"
-        message = "Global config backup root differs from runtime POBSYNC_BACKUP_ROOT."
+        message = "Global config backup root differs from the runtime backup root."
     return [
         SelfCheck(
             "Global config",