feat: multi-document upload + streaming LLM responses

feat: add session-based document management with 7-day auto-cleanup

feat(rag): add citation extraction with page numbers and source tracking

feat: Add multi-provider LLM support with UI model selector

Enterprise demo transformation: UI redesign, sample docs, Docker, auto-cleanup

OpenRouter Added, Rate Limitng Fixed

Clean snapshot for Hugging Face Space (no large files)