Fix extension panel manual add validation

[Jamesllllllllll]

Summary

Restore the Twitch panel manager manualAdd payload in the extension playlist route schema so moderator and broadcaster adds work again in production.

Related to #46.

What Changed

• added the missing manualAdd branch back to extensionPlaylistMutationSchema
• aligned the extension route validation with the payload the panel already sends for manager playlist adds
• added a regression test that covers the real panel manualAdd payload shape

How To Test

1. Open the Twitch panel as the broadcaster or a moderator.
2. Search for a song and click Add from the panel search results.
3. Confirm the request is added to the playlist instead of failing at /api/extension/playlist.
4. Run the checks locally:
   • npm run test -- tests/extension-playlist-mutation.test.ts tests/extension-panel.test.ts
   • npm run typecheck:ci
   • npm run check:prepush
   • npm run build:extension:panel

Screenshots Or UI Notes

No visible UI change. This patch restores a broken extension panel mutation path.

Checklist

• I tested the change locally
• I updated docs or examples if needed
• I called out any follow-up work or known limitations

[github-actions]

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