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/config_source.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+
+from typing import Any
+
+from pobsync.config.merge import build_effective_config
+
+from .config_repository import global_config_data, host_config_data
+
+
+class DjangoConfigSource:
+    def effective_config_for_host(self, host: str) -> dict[str, Any]:
+        return build_effective_config(global_config_data(), host_config_data(host))