fix: rewrite chapter detector with AI agent classification + RAG fallback

fix(engine): faster model switching on empty responses and BaseException handling

fix: Dockerfile installs pdf extra; test_web.py uses pytest fixture for isolation

fix: 3 issues — test patch target, test filesystem isolation, PLAN validate done; videopython import guard

fix: syntax error - indent contextlib import; shorten test comment

fix: 6 issues — tts-kitten optional, video model free warning, lifespan context, engine test isolation, PLAN updates, chromadb lock refresh

style: final lint cleanups - line breaks, import ordering, test client context

feat: comprehensive improvements (Groups 1-6)

feat(export): add Pygments syntax highlighting and copy button support

feat: model registry — live OpenRouter API with cache, free/vision filters

feat: v0.6.0 — EPUB export, GitHub Actions CI/release, gist publish

feat: v0.5.0 — richer RAG, DOCX export, parallel generation

feat: v0.4.0 — multi-file RAG, SQLite job store, better chapter detection

feat: v0.3.0 — smarter prompting, test suite, API stabilisation