docs: refresh stale product and API guidance

[davidhu2000]

Summary

• refresh billing, model, audio, and image guidance against current Magic Hour site/API
• fix stale routes, changelog URLs, media hosts, and invalid OpenAPI examples
• remove misleading free-credit, watermark, resolution, and input claims

Validation

• yarn check (with local bin path)
• OpenAPI JSON parse
• git diff --check

[davidhu2000]

Source counterpart in web-app: #13464. It updates fields.ts and code-sample generation so OpenAPI regeneration keeps these fixes.