refactor: use injected config sources for retention

Allow retention planning and pruning to use the same ConfigSource
abstraction as scheduled backups. This removes the remaining SQL-to-YAML
export dependency from Django backup runs with pruning, keeping YAML only
as a legacy CLI compatibility path.

src/pobsync_backend/management/commands/run_pobsync_backup.py

@@ -9,7 +9,6 @@ from django.utils import timezone
 
 from pobsync.commands.run_scheduled import run_scheduled
 from pobsync.paths import PobsyncPaths
-from pobsync_backend.config_repository import export_runtime_configs
 from pobsync_backend.config_source import DjangoConfigSource
 from pobsync_backend.models import BackupRun, HostConfig
 
@@ -33,11 +32,6 @@ class Command(BaseCommand):
         except HostConfig.DoesNotExist as exc:
             raise CommandError(f"Missing enabled HostConfig {host_name!r}") from exc
 
-        # Compatibility bridge: retention planning still reads runtime YAML.
-        # The backup run itself receives config directly from Django.
-        if options["prune"]:
-            export_runtime_configs(prefix=paths.home, host=host.host)
-
         run = BackupRun.objects.create(
             host=host,
             run_type=BackupRun.RunType.SCHEDULED,