Fixes: 1) Correct `name_ngram` search analyzer in `PostDocument` to `icu_query`;
Extra: 1) Update import paths and references following renaming of `BaseDocument` and `add_multilang_fields`.
Simplified field definitions and improved code consistency by consolidating and aligning styles. Added `prepare_*` methods to gracefully handle None values for fields. Enhanced multilingual support and streamlined query construction for better maintainability.
