feat(config): add ENABLE_CITATIONS environment variable

[Br1an67]

Description

Add an ENABLE_CITATIONS environment variable (default true) that controls whether citation references are included in query responses. When set to false, the reference list and reference_id fields are omitted from the LLM context, producing cleaner answers without source citations. Useful for Docker-based deployments where only .env configuration is available.

Related Issues

Fixes #2097

Changes Made

• lightrag/lightrag.py: Added enable_citations: bool field (default True) to the LightRAG dataclass
• lightrag/api/config.py: Parse ENABLE_CITATIONS env var via get_env_value()
• lightrag/api/lightrag_server.py: Wire enable_citations through to the LightRAG instance and debug log
• lightrag/operate.py: Conditionally skip reference list generation and reference_id in chunk context for both KG and naive query paths when citations are disabled
• env.example: Added commented ENABLE_CITATIONS entry

Checklist

• Changes tested locally
• Code reviewed
• Documentation updated (if necessary)
• Unit tests added (if applicable)

Additional Notes

Follows the same pattern as the existing ENABLE_LLM_CACHE / ENABLE_LLM_CACHE_FOR_EXTRACT configuration flags. Default is true so existing behavior is unchanged.