Implement /api/txStatus endpoint

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

- Call vm-sdk \`checkStatusCustomRequest()\`
- Cache with Cache API using 30s TTL
- **Query params**: \`staking_address\`, \`request_id\`, \`session_id\`
- **Returns**: status code (0=waiting, 1=processing, 2=failed, 3=success)

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

## Roadmap
Task 2.8