(refactor) Remove YAML config import and export path

Drop the pre-Django YAML import/export management commands and remove the
file-based config loader fallback from the backup and retention engines.

Keep the runtime config bridge backed by Django models, and add tests that
ensure engine operations require an explicit Django config source.

src/pobsync_backend/tests/test_run_scheduled_config_source.py

@@ -7,6 +7,7 @@ from unittest.mock import patch
 from django.test import SimpleTestCase
 
 from pobsync.commands.run_scheduled import run_scheduled
+from pobsync.errors import ConfigError
 from pobsync.rsync import RsyncResult
 
 
@@ -34,6 +35,10 @@ class FakeConfigSource:
 
 
 class RunScheduledConfigSourceTests(SimpleTestCase):
+    def test_requires_explicit_config_source(self) -> None:
+        with self.assertRaisesMessage(ConfigError, "A Django config source is required."):
+            run_scheduled(prefix=Path("/missing-prefix"), host="web-01", dry_run=True)
+
     def test_dry_run_uses_injected_config_source(self) -> None:
         with patch("pobsync.commands.run_scheduled.run_rsync") as run_rsync:
             run_rsync.return_value = RsyncResult(exit_code=0, command=["rsync", "--archive"])