loops: iteration timeline — GET /api/topology-runs?loop_id=X

Extend the topology-runs list route with an optional loop_id filter that
returns iterations for a single loop, newest-iteration-first. Adds the
iteration and finished_at columns to the summary (skip-null on the JSON
so compares stay compact). Backed by list_by_loop in the repo, which uses
the existing topology_runs_loop_idx partial index.

LoopsCanvas fetches the runs in parallel with the loop detail and renders
an iteration timeline card (iteration #, status pill, start time, duration,
run id prefix) between the graph section and the actions row.

crates/cm-api/src/routes/topology.rs

@@ -6,7 +6,7 @@
 use std::convert::Infallible;
 use std::time::Duration;
 
-use axum::extract::{Path, State};
+use axum::extract::{Path, Query, State};
 use axum::http::{HeaderMap, StatusCode};
 use axum::response::sse::{Event, KeepAlive, Sse};
 use axum::response::IntoResponse;
@@ -216,6 +216,8 @@ pub async fn run_swarm(
 }
 
 /// A saved/queued run, summarized (now includes lifecycle status + kind).
+/// `iteration` + `finished_at` populate for loop iterations / terminal runs;
+/// they're skipped from the JSON when null to keep the compares path compact.
 #[derive(Serialize)]
 pub struct RunSummary {
     pub id: String,
@@ -223,15 +225,40 @@ pub struct RunSummary {
     pub status: String,
     pub kind: String,
     pub created_at: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub iteration: Option<i32>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub finished_at: Option<String>,
+}
+
+/// Query params for `GET /api/topology-runs`. `loop_id` filters to a single
+/// loop's iterations, ordered newest-iteration-first.
+#[derive(Deserialize)]
+pub struct ListRunsQuery {
+    #[serde(default)]
+    pub loop_id: Option<Uuid>,
+    #[serde(default)]
+    pub limit: Option<i64>,
 }
 
 /// `GET /api/topology-runs` — recent runs for the workspace (compares + durable
-/// run jobs), newest first.
+/// run jobs), newest first. `?loop_id=X` filters to iterations of one loop,
+/// ordered by iteration DESC (uses `topology_runs_loop_idx`).
 pub async fn list_runs(
     State(state): State<AppState>,
     Authed(user): Authed,
+    Query(q): Query<ListRunsQuery>,
 ) -> Result<Json<Vec<RunSummary>>, ApiError> {
-    let rows = cm_db::repo::topology_runs::list_recent(&state.pool, user.workspace_id, 20).await?;
+    let limit = q.limit.filter(|n| *n > 0 && *n <= 200).unwrap_or(20);
+    let rows = match q.loop_id {
+        Some(loop_id) => {
+            cm_db::repo::topology_runs::list_by_loop(&state.pool, user.workspace_id, loop_id, limit)
+                .await?
+        }
+        None => {
+            cm_db::repo::topology_runs::list_recent(&state.pool, user.workspace_id, limit).await?
+        }
+    };
     let out = rows
         .into_iter()
         .map(|r| RunSummary {
@@ -240,6 +267,8 @@ pub async fn list_runs(
             status: r.status,
             kind: r.kind,
             created_at: r.created_at.format(&Rfc3339).unwrap_or_default(),
+            iteration: r.iteration,
+            finished_at: r.finished_at.and_then(|t| t.format(&Rfc3339).ok()),
         })
         .collect();
     Ok(Json(out))