[Feature] Track use or "people served"

## Description
It would be great to track use or "people served"
- Downloads for openly licensed works
- Listener statistics for openly licensed works or works that include openly licensed samples
- Purchases of openly licensed works
- ???

Questions:
- Which APIs might provide a direct or indirect indication of use?
- Are there any free non-API (non-machine-readable) data sources that are big enough to be worth including?
- ???

(This feature is blocked by #22)

Start by reviewing blocker #22, then examine the issue's proposed measures: downloads, listener statistics, and purchases. Research APIs and sufficiently large free non-machine-readable sources, and define the scope and criteria for tracking "people served" as the done state.

索引モデルが issue の本文から書いたものです。