(refactor) make Docker backup root static in Django setup

Remove backup_root from the normal Django global config form and display
the fixed container path /backups instead.

Always persist /backups from the setup form so Docker deployments do not
mix host paths with container paths.

Update tests and docs to clarify that the host backup directory is chosen
through the Docker mount, while Django always uses /backups internally.

src/pobsync_server/settings.py

@@ -89,3 +89,4 @@ STATIC_ROOT = os.getenv("POBSYNC_STATIC_ROOT", str(BASE_DIR / "var" / "static"))
 DEFAULT_AUTO_FIELD = "django.db.models.BigAutoField"
 
 POBSYNC_HOME = os.getenv("POBSYNC_HOME", "/opt/pobsync")
+POBSYNC_BACKUP_ROOT = "/backups"