Move ratingTiers to shared and fix the package entry point

ratingTiers is 148 identical lines duplicated in backend/src/helpers/ratingTiers.ts and frontend/src/helpers/getRatingStyles.ts. If a tier boundary or colour ever changes in one place and not the other, scores and their colours quietly disagree. This is exactly what the shared package is for.

• Move the tiers array and the getRatingStyles lookup into shared/src
• Both workspaces import from shared, delete the duplicates
• While in there: shared/package.json declares "main": "src/index.ts" but that file doesn't exist. Create a real index.ts re-exporting types and helpers so the package entry point isn't a lie