fix(env): guard checkout price config

[aiirvizionz]

Summary

• require positive integer env values for upload/cache/email limits instead of accepting zero, decimals, or partially numeric strings
• require positive USD price env values with at most two decimals before checkout pricing uses them
• fall back to the existing safe defaults when malformed values are configured

Validation

• CI=true pnpm --filter @aiornot/web exec vitest run lib/env.test.ts passed 3 tests
• CI=true pnpm --filter @aiornot/web exec vitest run lib/env.test.ts lib/sponsor-amount.test.ts lib/coinpay-blockchains.test.ts passed 12 tests