#37 shipped the telemetry retention task + model.DeleteTelemetryEventsBefore but
lacked a test for the deletion itself. Add TestDeleteTelemetryEventsBefore
(strict-< cutoff, retains at/after, idempotent) and migrate TelemetryEvent in the
model TestMain so it has a table.
Closes the "测试覆盖过期清理" DoD item of #43. Test-only; no behavior change.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>