feat(frontend): add optional title field to ingestion forms

feat: add optional custom title to document ingestion

fix: use sync Mock for count_tokens in integration test

refactor: move engine/session factory lifecycle to app.state via lifespan

docs: add demo video to README

docs: update ADRs to reflect current state and add planned improvements

fix: clarify that query rewriting only triggers with conversation history

docs: rewrite README for broader audience with architecture diagrams and metrics

fix: log conversation history and context chunks in Langfuse traces

feat: add opt-in Langfuse LLM observability via wrapper pattern

feat: add LLM-based query rewriting for conversational RAG retrieval

feat: add chat-style conversation UI with markdown rendering

feat: stream LLM answers to frontend via NDJSON

feat: add React frontend SPA with CORS support

refactor: extract shared prompt builder, fix exception types, and tighten schemas