support fetching user insights reports

[mrmauer]

Tests

TKTK

After PR

now you can...

let top_inviters_page = auth.user_insights().fetch_user_top_inviter_report(
    TopInviterReportInterval::ThirtyDays,
    ReportPagination {
        page_size: Some(10), // default 10 for all report fetches
        page_number: Some(0), // defaults 0 for all report fetches
    }
).await?;

let champions_page = auth.user_insights().fetch_user_champion_report(
    ChampionReportInterval::ThirtyDays,
    ReportPagination::default(),
).await?;

let user_churn_page = auth.user_insights().fetch_user_churn_report(
    ChurnReportInterval::SevenDays,
    ReportPagination::default(),
).await?;

let user_reengagement_page = auth.user_insights().fetch_user_reengagement_report(
    ReengagementReportInterval::Weekly,
    ReportPagination::default(),
).await?;

let org_churn_page = auth.user_insights().fetch_org_churn_report(
    ChurnReportInterval::ThirtyDays,
    ReportPagination::default(),
).await?;

let org_reengagement_page = auth.user_insights().fetch_org_reengagement_report(
    ReengagementReportInterval::Weekly,
    ReportPagination::default(),
).await?;

let org_growth_page = auth.user_insights().fetch_org_growth_report(
    GrowthReportInterval::ThirtyDays,
    ReportPagination::default(),
).await?;

let org_attrition_page = auth.user_insights().fetch_org_attrition_report(
    AttritionReportInterval::ThirtyDays,
    ReportPagination::default(),
).await?;

let timeseries_of_daily_signups = auth.user_insights().fetch_chart_metric_data(
    ChartMetric::Signups,
    FetchChartDataQuery {
        cadence: Some(ChartMetricCadence::Daily), // defaults to daily if not provided,
        start_date: Some("2026-01-01".to_string()), // defaults to 30 days ago if not provided
        ..Default::default(), // end_date defaults to today, but the most recently available results will be yesterday
    },
).await?;

Note: the start_date and end_date fields take raw strings to avoid the messiness of re-exporting time/chrono/other crates. This way the user can use whichever date/time tooling they want and we'll still format pre-fetch. A decent alternative is re-exporting a time::Date that's aliased...