Add digest summary endpoint

TL;DR

Adds a small Node service exposing a stubbed daily digest endpoint.

What changed?

Created a new digest.js file inside the sandbox-api directory that:

• Boots an HTTP server listening on port 4180
• Defines a stubbed digest payload with seven sample entries
• Implements a GET /digest route returning that payload as JSON

How to test?

1. Install dependencies: npm install
2. Start the service: node sandbox-api/digest.js
3. Hit the route: curl http://localhost:4180/digest
4. Confirm the response carries the seven stubbed digest entries

Why make this change?

This unblocks the dashboard work: the client team can build against a stable shape today instead of waiting on the ingestion service to land. The stub also pins the field names we expect each digest entry to carry, so the contract is settled before either side hardens.