docs(readme): restructure both READMEs and cover every current feature
Contents, highlights and quick start first, then features in the order a club uses them, then installation, decoders and development. Adds the start count-down, series hub, practice tooling, audio, OBS and security features, and removes outdated statements. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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