Add pregenerated chapter TTS support (audio field, player, generator script)

[Ruofei-personal]

Motivation

• Enable per-chapter pregenerated TTS audio playback and a workflow to generate audio files so chapters can include optional audio players without runtime TTS dependencies.

Description

• Added an optional audio field to chapter schemas in backend/app/models/schemas.py and to frontend types in frontend/src/lib/types.ts to carry audio URLs.
• Updated content loading in backend/app/services/content.py to include audio when building chapter listings.
• Render an inline audio player in the chapter page at frontend/src/app/books/[bookId]/chapters/[chapterSlug]/page.tsx when chapter.meta.audio is present.
• Documented the new content/books/<book-id>/audio/ directory and the workflow in README.md, including usage examples.
• Added a new script scripts/generate_tts.py that uses edge-tts to synthesize .mp3 files from chapter Markdown, writes them to content/books/<book-id>/audio/, and updates meta/<chapter>.json with the audio field; the script supports --chapter, --all, --voice, and --overwrite options.

Testing

• No automated tests were added or run as part of this change.

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Codex Task