refactor: runtime config, Q&A flow module, storage paths, and cleanup

docs: update all docs to reflect Qwen3-TTS architecture

Update docs to reflect story_downloader pipeline and 10-story corpus

Apply design critique fixes + add README

Add interruptible chunked streaming narration + mobile considerations PRD

Rewrite PRD for 2-day hackathon + add sprint.md

Update PRD: QWEN-TTS-0.6B, Gradio/HF Spaces, gr.Server, Stitch UI