[#8] Add Voucher clone — single-use redemption with atomic claim guarantee

[lincai505011-ops]

What

Adds the Voucher clone: single-use voucher redemption with atomic claim guarantee under concurrent load.

Design

• Pattern B: Atomic UPDATE … RETURNING — uses raw SQL UPDATE "Voucher" SET "redeemedBy" = $userId … WHERE "redeemedBy" IS NULL RETURNING * so Postgres serialises concurrent claimants on the row lock. No race, no 5xx.
• Empty RETURNING set → clean 404 (voucher not found) or 409 (already redeemed).
• Idempotent by construction: exactly one concurrent redeem wins, rest get 409.

Changes

File	Purpose
prisma/schema.prisma	Added Voucher model — code PK, value, redeemedBy (nullable), redeemedAt, createdAt
prisma/migrations/0001_voucher/migration.sql	Migration: CREATE TABLE
src/app.ts	Added POST /voucher (create) and POST /voucher/:code/redeem (atomic redeem) routes; added voucher counter to /metrics
prisma/seed.ts	Registered voucher clone in gallery
test/voucher.test.ts	5 tests: happy path, already-redeemed 409, not-found 404, validation 400, concurrent redeems invariant
probes/concurrency-voucher.mjs	Live concurrency probe: 20 concurrent redeems → exactly one 200, rest 409, zero 5xx

Invariant

20 concurrent redeems of one voucher → exactly one succeeds (200), the rest get clean 409 — zero 5xx.

Closes #8