feat: make Django configs drive backups and scheduling

Treat SQL-backed Django models as the source of truth for pobsync
configuration, exporting runtime YAML only as a compatibility layer for
the existing engine. Add a database-driven scheduler command, Docker
scheduler services, schedule run-state fields, and tests for scheduler,
config export, and retention behavior.

src/pobsync_backend/tests/test_retention.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+from datetime import datetime, timezone
+
+from django.test import SimpleTestCase
+
+from pobsync.retention import Snapshot, build_retention_plan
+
+
+class RetentionTests(SimpleTestCase):
+    def test_always_keeps_newest_snapshot(self) -> None:
+        snapshots = [
+            Snapshot("scheduled", "old", "/x/old", datetime(2026, 5, 18, tzinfo=timezone.utc), "success", None),
+            Snapshot("scheduled", "new", "/x/new", datetime(2026, 5, 19, tzinfo=timezone.utc), "failed", None),
+        ]
+
+        plan = build_retention_plan(
+            snapshots,
+            retention={"daily": 0, "weekly": 0, "monthly": 0, "yearly": 0},
+            now=datetime(2026, 5, 19, tzinfo=timezone.utc),
+        )
+
+        self.assertEqual(plan.keep, {"new"})
+        self.assertEqual(plan.reasons["new"], ["newest"])