Implement /api/estimateFees endpoint

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

- Call vm-sdk \`getEstimateFees()\`
- Cache with Cache API using 5min TTL
- **Query param**: \`token_count\`
- **Returns**: fee breakdown

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

## Roadmap
Task 2.11