fix: jitter_ms now measured in Rust at check start, excludes latency and return trip

apps/monitor/Cargo.toml

@@ -18,3 +18,4 @@ rustls-native-certs = "0.8"
 webpki-roots = "0.26"
 x509-parser  = "0.16"
 tokio-rustls = "0.26"
+chrono       = { version = "0.4", features = ["serde"] }

apps/monitor/src/runner.rs

@@ -57,6 +57,13 @@ pub async fn fetch_and_run(
 }
 
 async fn run_check(client: &reqwest::Client, monitor: &Monitor, scheduled_at: Option<String>) -> PingResult {
+    // 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()?;
+        let now = chrono::Utc::now();
+        Some((now - scheduled.with_timezone(&chrono::Utc)).num_milliseconds())
+    });
+
     let start = Instant::now();
 
     // Check cert expiry for HTTPS URLs
@@ -97,6 +104,7 @@ async fn run_check(client: &reqwest::Client, monitor: &Monitor, scheduled_at: Op
         Err(e) => PingResult {
             monitor_id: monitor.id.clone(),
             scheduled_at,
+            jitter_ms,
             status_code: None,
             latency_ms: Some(latency_ms),
             up: false,
@@ -144,6 +152,7 @@ async fn run_check(client: &reqwest::Client, monitor: &Monitor, scheduled_at: Op
             PingResult {
                 monitor_id: monitor.id.clone(),
                 scheduled_at,
+                jitter_ms,
                 status_code: Some(status),
                 latency_ms: Some(latency_ms),
                 up,

apps/monitor/src/types.rs

@@ -19,6 +19,7 @@ pub struct Monitor {
 pub struct PingResult {
     pub monitor_id:       String,
     pub scheduled_at:     Option<String>,
+    pub jitter_ms:        Option<i64>,
     pub status_code:      Option<u16>,
     pub latency_ms:       Option<u64>,
     pub up:               bool,