docs: refresh README, FAQ, package metadata, and repo social preview

[pietechsolution]

Documentation and metadata pass on this server SDK repo. No library code changes.

• README: clearer H1 and intro describing the server SDK as the trusted side of a reserved-seating integration, plus a short navigation strip to the registry page, server SDK docs, the product site, and the buyer-side SDK.
• Install section states the real published coordinates and the current release.
• New FAQ covering booking from this language, the server vs buyer SDK boundary, how holds work server-side, and using your own payment provider. Every statement checked against this repository's source and the published docs.
• Ecosystem table normalised to match the other SeatLayer SDK repos.
• Package manifest description/keywords refreshed.
• Added .github/social-preview.png. It still has to be uploaded manually in Settings, the API cannot set it.

All links in the README were checked and resolve (npm package pages return 403 to command-line requests, which is their bot protection, and open fine in a browser).