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_config_repository.py

@@ -0,0 +1,47 @@
+from __future__ import annotations
+
+import tempfile
+from pathlib import Path
+
+from django.test import TestCase
+
+from pobsync.config.load import load_global_config, load_host_config
+from pobsync_backend.config_repository import export_runtime_configs
+from pobsync_backend.models import GlobalConfig, HostConfig
+
+
+class ConfigRepositoryTests(TestCase):
+    def test_exports_database_configs_to_engine_yaml(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            prefix = Path(tmp)
+            GlobalConfig.objects.create(
+                name="default",
+                backup_root="/backups",
+                pobsync_home=str(prefix),
+                data={
+                    "backup_root": "/ignored",
+                    "pobsync_home": "/ignored",
+                    "retention_defaults": {"daily": 7, "weekly": 4, "monthly": 3, "yearly": 1},
+                },
+            )
+            HostConfig.objects.create(
+                host="web-01",
+                address="web-01.example.test",
+                config={
+                    "host": "ignored",
+                    "address": "ignored",
+                    "retention": {"daily": 7, "weekly": 4, "monthly": 3, "yearly": 1},
+                    "includes": [],
+                    "excludes_add": ["/tmp/***"],
+                },
+            )
+
+            written = export_runtime_configs(prefix=prefix, host="web-01")
+
+            self.assertEqual(len(written), 2)
+            global_cfg = load_global_config(prefix / "config" / "global.yaml")
+            host_cfg = load_host_config(prefix / "config" / "hosts" / "web-01.yaml")
+            self.assertEqual(global_cfg["backup_root"], "/backups")
+            self.assertEqual(global_cfg["pobsync_home"], str(prefix))
+            self.assertEqual(host_cfg["host"], "web-01")
+            self.assertEqual(host_cfg["address"], "web-01.example.test")