Add Chapters API module

[guidefari]

Summary

Add Chapters API module for audiobook chapter endpoints.

Context

Spotify API has dedicated chapter endpoints (/chapters/{id}, /chapters). This is a companion to #34 (Audiobooks) — chapters are a separate resource in the Spotify API.

Upstream spotify-web-api-ts exposes getChapter() and getChapters().

Acceptance criteria

• Chapter model types and schemas
• Chapters service with getChapter() and getChapters()
• Exported from index.ts

Related

PRD #9, #34

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Migrated from guidefari/spotify-effect#49