Gradio chat with sources and logging

Build a chat app: the answer with clickable sources (file, page) under it, a thumbs up/down button, streaming answers, and logging of every question, retrieved chunks, answer, time and cost (a CSV, or a tracing tool like Langfuse). Build the vector database once at start-up, not per question. Publish on Spaces (with permission for the documents) and write the README with the results table.