Fix WS recv race + session recycle 20 with async overlap + Modal deploy

Sync backend with LN-TTS: fix Chapter 1 scraper bug, add sentence offsets, improve error handling, update README

Sentence-chunked WS audio with smoothing

Improve chapter parsing, add sentence_total and voice to chapter_info

Sync backend features: novel_details, realtime flag, portable PORT, Azure deploy docs

initial commit