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/tests/test_django_config_source.py

@@ -0,0 +1,48 @@
+from __future__ import annotations
+
+from django.test import TestCase
+
+from pobsync_backend.config_source import DjangoConfigSource
+from pobsync_backend.models import GlobalConfig, HostConfig
+
+
+class DjangoConfigSourceTests(TestCase):
+    def test_returns_effective_config_from_database(self) -> None:
+        GlobalConfig.objects.create(
+            name="default",
+            backup_root="/backups",
+            pobsync_home="/opt/pobsync",
+            data={
+                "backup_root": "/ignored",
+                "pobsync_home": "/ignored",
+                "ssh": {"user": "root", "port": 22, "options": []},
+                "rsync": {
+                    "binary": "rsync",
+                    "args": ["--archive"],
+                    "timeout_seconds": 0,
+                    "bwlimit_kbps": 0,
+                    "extra_args": ["--numeric-ids"],
+                },
+                "defaults": {"source_root": "/", "destination_subdir": ""},
+                "excludes_default": ["/proc/***"],
+                "retention_defaults": {"daily": 7, "weekly": 4, "monthly": 3, "yearly": 1},
+            },
+        )
+        HostConfig.objects.create(
+            host="web-01",
+            address="web-01.example.test",
+            config={
+                "retention": {"daily": 7, "weekly": 4, "monthly": 3, "yearly": 1},
+                "excludes_add": ["/tmp/***"],
+                "includes": [],
+                "rsync": {"extra_args": ["--delete"]},
+            },
+        )
+
+        cfg = DjangoConfigSource().effective_config_for_host("web-01")
+
+        self.assertEqual(cfg["backup_root"], "/backups")
+        self.assertEqual(cfg["host"], "web-01")
+        self.assertEqual(cfg["address"], "web-01.example.test")
+        self.assertEqual(cfg["excludes_effective"], ["/proc/***", "/tmp/***"])
+        self.assertEqual(cfg["rsync"]["args_effective"], ["--archive", "--numeric-ids", "--delete"])