Add usage/popularity metrics

### Description
The design supports a "Most used" section, but the application needs a real metric before ranking bots/framework/server/etc.

### Research
- Can room usage be discovered reliably?
- Should maintainers submit usage statistics?
- Could bots expose standardized telemetry?
- Should GitHub stars/downloads be used only as secondary signals?

### Acceptance criteria
- Document definition of "usage"
- Avoid misleading rankings
- Define update/freshness strategy
- Add API field only after metric is trustworthy

No files or tests are named in the issue. Start by investigating reliable room-usage, maintainer-submitted, bot-telemetry, and secondary GitHub signals, then define usage, ranking safeguards, and freshness requirements. Done means the metric is documented, misleading rankings are addressed, an update strategy is defined, and the API field is deferred until the metric is trustworthy.

Written by the indexing model from the issue text.