refactor(idm): share directory dependency resolution

Release v0.1.26. Coordinated integrity review: GovOPlaN/govoplan-core#298.

src/govoplan_idm/backend/api/v1/directory_dependencies.py

@@ -0,0 +1,20 @@
+"""IDM route dependencies; resolve the current optional Core capability per call."""
+
+from fastapi import HTTPException, status
+
+from govoplan_core.core.identity import CAPABILITY_IDENTITY_DIRECTORY, IdentityDirectory
+
+
+def require_identity_directory(registry: object | None) -> IdentityDirectory:
+    if registry is None or not registry.has_capability(CAPABILITY_IDENTITY_DIRECTORY):
+        raise HTTPException(
+            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+            detail="Identity directory is unavailable",
+        )
+    capability = registry.require_capability(CAPABILITY_IDENTITY_DIRECTORY)
+    if not isinstance(capability, IdentityDirectory):
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail=f"Invalid capability: {CAPABILITY_IDENTITY_DIRECTORY}",
+        )
+    return capability