feat(backend): demo highlights, boost persistence, scoring & rate limiting

[openwork-hackathon]

Backend Polish Sprint — ghost_llm (Contract)

Closes #100, supports #111

Changes:

1. Demo Mode Backend (Issue #111)

• GET /api/demo/highlights — returns top 10 agents for cinematic auto-pan
• Selection criteria: reputation, tips, unique tags, activity score
• Returns coordinates + highlight reason

2. Boost Persistence (Issue #100)

• src/lib/storage.ts — file-based JSON persistence for boosts/tips
• POST /api/boost and POST /api/tip endpoints store to data/boosts.json
• Replaces in-memory stubs

3. Improved Scoring Heuristics

• Recency decay (0.5 factor every 48h)
• Boost multiplier (up to 1.5x)
• Interaction diversity bonus (+5 per unique source, cap +25)
• Badge count bonus (+10 per badge)

4. Rate Limiting

• src/lib/rate-limit.ts — shared utility
• 10 req/IP/min on POST /api/tip and /api/boost
• Returns 429 + Retry-After header

Files Added/Modified:

• src/app/api/demo/highlights/route.ts (new)
• src/app/api/boost/route.ts (new)
• src/app/api/tip/route.ts (modified)
• src/lib/storage.ts (new)
• src/lib/rate-limit.ts (new)
• src/lib/openwork.ts (enhanced scoring)