Implement /api/resolveHandle endpoint

## Description
Implement the \`/api/resolveHandle\` Pages Function endpoint.

- Resolve ADA Handle (\$handle) to a stake address
- Cache with Cache API using 1hr TTL (handles rarely change ownership)
- **Query param**: \`handle\`
- **Returns**: resolved stake address

## Files
- Create \`functions/api/resolveHandle.ts\`

## Roadmap
Task 2.15