refactor: inject config sources into scheduled backups

Introduce a ConfigSource interface so scheduled backups no longer need
to load host configuration directly from runtime YAML. Add a Django-backed
config source for SQL-driven backup runs, keep file-based config as the
CLI default, and make scheduled prune execution actually apply retention
after successful runs.

src/pobsync_backend/management/commands/run_pobsync_backup.py

@@ -10,6 +10,7 @@ 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
 
 
@@ -32,7 +33,10 @@ class Command(BaseCommand):
         except HostConfig.DoesNotExist as exc:
             raise CommandError(f"Missing enabled HostConfig {host_name!r}") from exc
 
-        export_runtime_configs(prefix=paths.home, host=host.host)
+        # 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,
@@ -49,6 +53,7 @@ class Command(BaseCommand):
                 prune=bool(options["prune"]),
                 prune_max_delete=int(options["prune_max_delete"]),
                 prune_protect_bases=bool(options["prune_protect_bases"]),
+                config_source=DjangoConfigSource(),
             )
         except Exception as exc:
             run.status = BackupRun.Status.FAILED