refactor(autocomplete): remove old CSV fallback, use TagFTSIndex exclusively

Remove all autocomplete.txt parsing logic and fallback code, simplifying
the service to use only TagFTSIndex for Danbooru/e621 tag search
with category filtering.

- Remove WordEntry dataclass and _words_cache, _file_path attributes
- Remove _determine_file_path(), get_file_path(), load_words(), save_words(),
  get_content(), _parse_csv_content() methods
- Simplify search_words() to only use TagFTSIndex, always returning
  enriched results with {tag_name, category, post_count}
- Remove GET/POST /api/lm/custom-words endpoints (unused)
- Keep GET /api/lm/custom-words/search for frontend autocomplete
- Rewrite tests to focus on TagFTSIndex integration

This reduces code by 446 lines and removes untested pysssss plugin
integration. Feature is unreleased so no backward compatibility needed.

py/routes/misc_route_registrar.py

@@ -42,8 +42,6 @@ MISC_ROUTE_DEFINITIONS: tuple[RouteDefinition, ...] = (
     RouteDefinition("GET", "/api/lm/metadata-archive-status", "get_metadata_archive_status"),
     RouteDefinition("GET", "/api/lm/model-versions-status", "get_model_versions_status"),
     RouteDefinition("POST", "/api/lm/settings/open-location", "open_settings_location"),
-    RouteDefinition("GET", "/api/lm/custom-words", "get_custom_words"),
-    RouteDefinition("POST", "/api/lm/custom-words", "update_custom_words"),
     RouteDefinition("GET", "/api/lm/custom-words/search", "search_custom_words"),
 )