fix: use monitor-side checked_at timestamp instead of coordinator arrival time

apps/monitor/src/runner.rs

@@ -100,6 +100,7 @@ pub async fn fetch_and_run(
                 Ok(r) => r,
                 Err(_) => PingResult {
                     monitor_id: monitor.id.clone(),
+                    checked_at: Some(chrono::Utc::now().to_rfc3339()),
                     scheduled_at: scheduled_at_iso.clone(),
                     jitter_ms: None,
                     status_code: None,
@@ -125,6 +126,9 @@ pub async fn fetch_and_run(
 }
 
 async fn run_check(client: &reqwest::Client, monitor: &Monitor, scheduled_at: Option<String>, region: &str, run_id: &str) -> PingResult {
+    // Record when the check actually started (used as checked_at in the ping)
+    let checked_at = chrono::Utc::now().to_rfc3339();
+
     // Compute jitter: how late we actually started vs when we were scheduled
     let jitter_ms: Option<i64> = scheduled_at.as_deref().and_then(|s| {
         let scheduled = chrono::DateTime::parse_from_rfc3339(s).ok()?;
@@ -175,6 +179,7 @@ async fn run_check(client: &reqwest::Client, monitor: &Monitor, scheduled_at: Op
             debug!("{} check error: {e}", monitor.url);
             PingResult {
                 monitor_id: monitor.id.clone(),
+                checked_at: Some(checked_at.clone()),
                 scheduled_at,
                 jitter_ms,
                 status_code: None,
@@ -236,6 +241,7 @@ async fn run_check(client: &reqwest::Client, monitor: &Monitor, scheduled_at: Op
 
             PingResult {
                 monitor_id: monitor.id.clone(),
+                checked_at: Some(checked_at),
                 scheduled_at,
                 jitter_ms,
                 status_code: Some(status),

apps/monitor/src/types.rs

@@ -33,6 +33,7 @@ pub struct Monitor {
 #[derive(Debug, Serialize)]
 pub struct PingResult {
     pub monitor_id:       String,
+    pub checked_at:       Option<String>,
     pub scheduled_at:     Option<String>,
     pub jitter_ms:        Option<i64>,
     pub status_code:      Option<u16>,