feat(validation): add currency code enum for input validation

[zinodict121]

Closes #89

---

Summary

Replaces plain z.string() validation on currency fields with a typed CurrencyCode enum, improving runtime error messages and enabling IDE autocompletion.

Changes

• Defined CurrencyCode as z.enum(['USDC', 'EURT', 'NGN']) in shared/validation/currency.ts
• Exported the CurrencyCode TypeScript type in shared/validation/index.ts
• Updated CreatePaymentBody.asset to use CurrencyCode
• Updated CreateSettlementBody.asset to use CurrencyCode
• Updated FX quote schema from/to fields to use CurrencyCode
• Note: Resolved existing git merge conflicts and strict-mode typing errors (latestLedgerCursor) in services/indexer/src/index.ts to restore a green testing baseline.

Why

Currency codes were previously validated as arbitrary strings, meaning invalid values like "USD" or "usd" would pass schema validation silently. The enum provides:

• Strict validation with descriptive error messages
• Full TypeScript type safety (CurrencyCode type)
• IDE autocompletion on all currency fields
• A single place to add new currencies in the future

Testing

• All existing tests pass: pnpm test
• TypeScript compiles without errors: pnpm -r type-check

Extensibility

To add a new currency, update the enum in shared/validation/currency.ts:

```ts
export const CurrencyCode = z.enum(['USDC', 'EURT', 'NGN', 'NEW_CURRENCY']);
```

No other changes are required.

Checklist

• Code follows project conventions
• No unrelated files changed (aside from resolving the broken baseline)
• Tests pass
• TypeScript compiles clean

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[therealjhay]

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[zinodict121]

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