Add inside-athletics write-up

[aahiltn]

Summary

• Adds a one-page technical retrospective for GenerateNU/inside-athletics (spring2026 cohort) to projects/spring2026/inside-athletics.md.
• Cold read of the repo: commit history, PR history, and source (backend Go handlers + frontend Next.js routes/hooks).
• Covers auth (Supabase JWT + JWKS), RBAC middleware, S3 presigned uploads, Stripe Checkout/webhooks + role sync, the premium paywall (and a gap found where GET routes aren't permission-gated), the 10-step onboarding wizard, and a repeated infinite-scroll pattern across 4 frontend pages.

Test plan

• Verified contributor list against git shortlog + GitHub contributors API (bots excluded).
• Verified cohort guess (spring2026) against first/last commit dates.
• Checked projects/spring2026/ for a naming collision — none found.
• N/A — documentation-only change, no code to run.

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