Feat: validate currency iso 4217

[Ezeh20]

Summary

• Registers a custom iso4217 validator (/^[A-Z]{3}$/) in src/utils/validator.ts, following the same pattern as bcp47 and ianaTimezone
• Replaces the weak min:3,max:3 rule on currencyPreference in src/utils/profileValidators.ts with iso4217
• Updates docs/USER_PROFILE_PREFERENCES_API.md to document the ISO 4217 policy explicitly
• Adds two new tests in src/controllers/__tests__/preferences.controller.test.ts:
  • Valid codes (USD, EUR, GBP, JPY, NGN) → 202
  • Invalid values (usd, US, USDD, 123, abc, u s) → 422 with currencyPreference error

Test plan

• Run src/controllers/__tests__/preferences.controller.test.ts — both new currency tests pass
• POST /api/preferences with currencyPreference: "usd" returns 422
• POST /api/preferences with currencyPreference: "USD" returns 202
• POST /api/preferences with currencyPreference: "123" returns 422

Closes #152