Add leaderboard feature with podium-style redesign

[BOOPATB]

What

Adds a global leaderboard for GitFut:

• Redis sorted-set storage (lib/leaderboard.ts) — record/get/rank
• GET /api/leaderboard endpoint
• /leaderboard page with a podium treatment for the top 3 (TOTW-reveal
  style: 2nd–1st–3rd, elevated, staggered pop-in) and tier-tinted glow
  borders throughout, using the existing RESULT_THEME tokens
• Ambient "flood" background matching the home page

Also included

• Fix for a duplicate buildFresh + duplicate import bug in lib/scout.ts
• Minor updates to AppShell.tsx / lib/redis.ts needed to support the
  leaderboard writes

Notes

• Founder-card-specific accents aren't wired in yet — leaderboard entries
  only carry finish, not full card data, so founder rows use the
  founder tier color from RESULT_THEME rather than the fully custom
  per-card accent.

[BOOPATB]

Noticed #58 also implements a global leaderboard, and it looks solid — search, pagination, tests, and a graceful fallback when Redis isn't configured, which mine doesn't have.
This PR was mainly about the visual treatment rather than the underlying feature: a podium layout for the top 3 (elevated, staggered pop-in), tier-tinted glow borders throughout using the existing RESULT_THEME tokens, and the flood ambient background matching the home page.
If #58 lands first, happy to rework this as a follow-up that reskins its /leaderboard page with the podium/glow treatment instead — whichever's more useful. Let me know either way.