Advertise format-specific availability statistics

[snapetech]

Summary

• Advertise the existing ebook and audiobook availability statistics in SeerrNG capability discovery.
• Update the integration guide and OpenAPI schema, with a regression assertion and release note.

Verification

• Frontend build, typecheck, and React Router context check passed.
• Chaptarr.Core.Test: 3,343 passed, 3 skipped, 0 failed. The skipped cases require the Linux Chromium setup used by CI.
• Release solution build and Console/Updater publish passed with 0 warnings and 0 errors.
• Release-note validation and preview passed.

CI still needs to run the repository's Linux, Windows, and macOS matrix.

AI assistance: Codex.