fix(referrals): validate positive cent amounts

[rissrice2105-agent]

Summary

• requires referral purchase amounts to be positive safe integer cents
• rejects zero, negative, fractional, unsafe, non-number, and missing values with HTTP 400
• prevents invalid commission and discount amounts from being persisted
• adds route-level regression coverage that verifies no usage is saved

Validation

• vitest run tests/referrals.test.ts (45 tests)
• full suite: 254 tests
• tsc --noEmit
• tsup build
• git diff --check

Closes #4