feat: add script for scanning for missing env vars in docs

[cheahjs]

Walks the AST for the backend code for calls to os.environ/os.getenv, and compares it with what is currently documented.

Run with python3 scripts/scan-missing-env-vars.py <git_ref>, for example python3 scripts/scan-missing-env-vars.py v0.3.20

Example output:

$ scripts/scan-missing-env-vars.py v0.3.20
Scanning git ref: v0.3.20

Environment variables accessed but not documented:
AUDIO_TTS_API_KEY
AUDIO_TTS_SPLIT_ON
AUTOMATIC1111_API_AUTH
COMFYUI_WORKFLOW
CONTENT_EXTRACTION_ENGINE
CORS_ALLOW_ORIGIN
DEFAULT_LOCALE
ENABLE_ADMIN_CHAT_ACCESS
ENABLE_MESSAGE_RATING
ENABLE_SEARCH_QUERY
FONTS_DIR
FROM_INIT_PY
FUNCTIONS_DIR
GLOBAL_LOG_LEVEL
RAG_FILE_MAX_COUNT
RAG_FILE_MAX_SIZE
SEARCHAPI_API_KEY
SEARCHAPI_ENGINE
TIKA_SERVER_URL
TOOLS_DIR
USER_PERMISSIONS_CHAT_EDITING
USER_PERMISSIONS_CHAT_TEMPORARY
WEBUI_JWT_SECRET_KEY

Environment variables documented but not accessed:
ENABLE_LITELLM
LITELLM_PROXY_HOST
LITELLM_PROXY_PORT
RAG_WEB_SEARCH_DOMAIN_FILTER_LIST
SEARCH_QUERY_PROMPT_LENGTH_THRESHOLD