feat: add FastAPI manager API compatibility baseline

main/manager-api-fastapi/app/services/java_validation.py

@@ -0,0 +1,21 @@
+from __future__ import annotations
+
+from functools import lru_cache
+from pathlib import Path
+
+from app.core.config import get_settings
+from app.core.i18n import LANGUAGE_FILES, _load_properties, resolve_language
+
+
+@lru_cache(maxsize=16)
+def _validation_messages(language: str, directory: str) -> dict[str, str]:
+    root = Path(directory)
+    values = _load_properties(root / "validation.properties")
+    localized = LANGUAGE_FILES[language].replace("messages_", "validation_")
+    values.update(_load_properties(root / localized))
+    return values
+
+
+def validation_message(key: str, accept_language: str | None) -> str:
+    language = resolve_language(accept_language)
+    return _validation_messages(language, str(get_settings().i18n_dir)).get(key, key)