(feature) Add host backup preflight gates

Introduce a host preflight layer that separates dry-run blockers from real backup blockers.
Show the effective per-host backup configuration in Django before queueing a run.

Block real backup queueing when failed host checks remain, while still allowing dry-runs
when only local storage preparation is missing.

src/pobsync_backend/config_repository.py

@@ -77,6 +77,14 @@ def _host_yaml_data(host_config: HostConfig) -> dict[str, Any]:
     return validate_dict(data, HOST_SCHEMA, path="host")
 
 
+def global_config_object_data(global_config: GlobalConfig) -> dict[str, Any]:
+    return _global_yaml_data(global_config)
+
+
+def host_config_object_data(host_config: HostConfig) -> dict[str, Any]:
+    return _host_yaml_data(host_config)
+
+
 def global_config_data(name: str = "default") -> dict[str, Any]:
     try:
         global_config = GlobalConfig.objects.get(name=name)