refactor: use injected config sources for retention

Allow retention planning and pruning to use the same ConfigSource
abstraction as scheduled backups. This removes the remaining SQL-to-YAML
export dependency from Django backup runs with pruning, keeping YAML only
as a legacy CLI compatibility path.

src/pobsync_backend/tests/test_retention_config_source.py

@@ -0,0 +1,53 @@
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from pathlib import Path
+from tempfile import TemporaryDirectory
+
+from django.test import SimpleTestCase
+
+from pobsync.commands.retention_plan import run_retention_plan
+from pobsync.util import write_yaml_atomic
+
+
+class FakeConfigSource:
+    def __init__(self, backup_root: str) -> None:
+        self.backup_root = backup_root
+
+    def effective_config_for_host(self, host: str) -> dict:
+        return {
+            "backup_root": self.backup_root,
+            "host": host,
+            "address": "example.test",
+            "retention": {"daily": 1, "weekly": 0, "monthly": 0, "yearly": 0},
+        }
+
+
+class RetentionConfigSourceTests(SimpleTestCase):
+    def test_retention_plan_uses_injected_config_source(self) -> None:
+        with TemporaryDirectory() as tmp:
+            root = Path(tmp) / "backups"
+            snap_dir = root / "web-01" / "scheduled" / "20260519-021500Z__ABCDEFGH"
+            meta_dir = snap_dir / "meta"
+            meta_dir.mkdir(parents=True)
+            write_yaml_atomic(
+                meta_dir / "meta.yaml",
+                {
+                    "status": "success",
+                    "started_at": datetime(2026, 5, 19, 2, 15, tzinfo=timezone.utc)
+                    .isoformat()
+                    .replace("+00:00", "Z"),
+                },
+            )
+
+            plan = run_retention_plan(
+                prefix=Path("/missing-prefix"),
+                host="web-01",
+                kind="scheduled",
+                protect_bases=False,
+                config_source=FakeConfigSource(str(root)),
+            )
+
+        self.assertTrue(plan["ok"])
+        self.assertEqual(plan["keep"], ["20260519-021500Z__ABCDEFGH"])
+        self.assertEqual(plan["delete"], [])