feat: port medialog module (media event logging)

[tripodsan]

Background

helix-admin has a dedicated media event logging module (src/medialog/) not yet in helix-api-service.

What it does

• POST: add up to 10 media log entries per request; auto-tags with current user email
• GET: query logs with time-range filtering (from/to/since), pagination with nextToken
• Generates link headers for pagination
• Separate from the general audit log

Key files in helix-admin

• src/medialog/handler.js - routes GET and POST
• src/medialog/add.js - batch enqueue entries
• src/medialog/query.js - time-based filtering, pagination, link generation

helix-admin PRs

• adobe/helix-admin#3450 - add media log handler
• adobe/helix-admin#3478 - add medialog triggers
• adobe/helix-admin#3509 - log media unlink operations to medialog
• adobe/helix-admin#3486 - handle medialog updates in a backward-compatible way

What to do

Port the medialog module to helix-api-service. Register a new route in src/index.js. Note: this is separate from src/log/ (general audit log).