Add supporter metadata and subtle listing badge

[antiero]

Motivation

• Provide first-class, optional sponsorship metadata so supporters can be recognised in the public directory without affecting editorial behaviour.
• Mark PlayDrumsOnline as an active supporter for the public record while keeping listing order and search unchanged.

Description

• Add SponsorTier and SponsorInfo types and an optional sponsor?: SponsorInfo field on Site in src/types.ts.
• Mark PlayDrumsOnline with sponsor: { active: true, tier: 'supporter', since: '2026-05-11' } in src/data.ts.
• Render a subtle Supporter badge when site.sponsor?.active is true in both src/components/SiteCard.tsx and src/components/SiteRow.tsx.
• Add a short ## Sponsorship policy blurb to README.md clarifying that sponsorship does not guarantee inclusion, ranking, endorsement, or editorial preference.

Testing

• Ran npm run lint (tsc --noEmit) which completed successfully.
• Ran npm run build (vite build) which completed successfully and produced a production build.
• No changes to sorting, filtering, or ranking logic were introduced and non-sponsored entries remain unaffected.

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Codex Task