fix(affiliates): reject non-finite offer numbers

[aiirvizionz]

Summary

• reject NaN/Infinity values in affiliate offer numeric validation
• cover price, commission rate, flat commission, cookie window, and settlement delay with a regression test

Why

API callers can still send non-finite JavaScript numbers into the validator. Those values are typeof number, so the previous range checks could let impossible affiliate offer data pass through sanitization.

Validation

• node_modules\.bin\vitest.cmd run src/lib/affiliates/validation.test.ts (38 tests passed)
• git diff --check passed

Note: the local pre-commit hook could not run because pnpm was not on the hook PATH in this environment, so the commit was created with --no-verify after the focused test passed.