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_repository.py

@@ -31,6 +31,22 @@ def _host_yaml_data(host_config: HostConfig) -> dict[str, Any]:
     return validate_dict(data, HOST_SCHEMA, path="host")
 
 
+def global_config_data(name: str = "default") -> dict[str, Any]:
+    try:
+        global_config = GlobalConfig.objects.get(name=name)
+    except ObjectDoesNotExist as exc:
+        raise ConfigRepositoryError(f"Missing GlobalConfig {name!r}") from exc
+    return _global_yaml_data(global_config)
+
+
+def host_config_data(host: str) -> dict[str, Any]:
+    try:
+        host_config = HostConfig.objects.get(host=host, enabled=True)
+    except ObjectDoesNotExist as exc:
+        raise ConfigRepositoryError(f"Missing enabled HostConfig {host!r}") from exc
+    return _host_yaml_data(host_config)
+
+
 def export_global_config(prefix: Path, name: str = "default") -> Path:
     try:
         global_config = GlobalConfig.objects.get(name=name)