docs: sync docs with actual stack

[catomean]

Docs truth-sweep: every claim below was verified against repo ground truth (package.json, CI workflow, src/lib/bonus-codes.ts, src/lib/db/schema.ts, scripts/db/seed.ts) before editing. npm→pnpm and Prisma→Drizzle migration in the docs was already complete; the drift found was elsewhere.

README.md

• Badges + tech-stack table claimed Next.js 15 / TypeScript 5 → package.json has next ^16.3.3, typescript ^6.0.3 → now 16 / 6
• Prerequisites claimed "Node.js 18+" → engines.node >=20, CI runs Node 24 → now "20+ (CI runs on 24)"
• Setup said cp .env.example .env.local → no .env.example exists (.env* is gitignored) → replaced with "create .env.local with …" comment
• Claimed 30-day bonus expiry → getBonusExpiryDate() adds one calendar month → now "one-month"

.claude/CLAUDE.md

• Tech table: Next.js 15 → 16
• Bonus rules claimed "20% of repair cost (max CHF 50)" and "expire after 1 year" → calculateBonusAmount() returns fixed 100, expiry is one month → fixed
• ShopCategory shown as a 10-value union → DB enum has exactly ELECTRONICS | CLOTHING | SHOES → fixed
• "Default Users (After Seeding)" table (admin/customer logins) → scripts/db/seed.ts creates shops only, zero users → replaced with truthful Seed Data note
• Insert example used amount: 50 and a nonexistent generateCode() → now uses the real helpers
• Last Updated stamp → 2026-09-04

docs/how-it-works-implementation.md

• Stack claim "Next.js 15 and React 18" → 16 / 19 (React ^19.0.0); header Last Modified updated per the doc's own standard
• Its "30 Tage" statements were left alone: they accurately describe the page copy (Gültig für 30 Tage in src/app/how-it-works/page.tsx)

pnpm run format:check passes.

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