Implement discovery ranking algorithm

[iamblake21]

Closes #3

Summary

• adds a reusable discovery ranking module with weighted score factors for funding, reward, recency, competition, and tag diversity
• updates /discovery to show rank, total score, and factor breakdown for each bounty
• adds node:test coverage for ranking order, score factors, non-negative scores, and deterministic tie-breaking
• includes desktop/mobile screenshots plus a short GIF demo under screenshot/
• fixes the build-blocking validation error state in CreateBountyForm so the app can type-check successfully

Demo

• screenshot/discovery-desktop.png
• screenshot/discovery-mobile.png
• screenshot/discovery-demo.gif

Verification

• npm test
• npm run build

Note: build completes with the existing Next.js warning about in src/components/leaderboard.tsx.