fix: reduce usage stats polling payload

[bbbugg]

Summary

• prevent the status page from starting duplicate polling loops
• default usage statistics to the last 30 days and send startTime/endTime with each request
• filter usage records by startedAt on the server
• ignore stale responses when the selected time range changes

Verification

• npm run build:ui
• node test/usage-stats-readers.test.js
• node test/usage-stats-import-started-at.test.js

Fixes #260