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-db/src/repo/topology_runs.rs

@@ -10,13 +10,17 @@ use uuid::Uuid;
 
 use crate::DbError;
 
-/// A row summary for the recent-runs list.
+/// A row summary for the recent-runs list. `iteration` and `finished_at`
+/// are populated for loop iterations and for terminal runs respectively;
+/// `None` for compares or still-in-flight runs.
 pub struct TopologyRunSummary {
     pub id: Uuid,
     pub task: String,
     pub status: String,
     pub kind: String,
     pub created_at: OffsetDateTime,
+    pub iteration: Option<i32>,
+    pub finished_at: Option<OffsetDateTime>,
 }
 
 /// A full saved comparison run.
@@ -271,7 +275,8 @@ pub async fn list_recent(
     limit: i64,
 ) -> Result<Vec<TopologyRunSummary>, DbError> {
     let rows = sqlx::query!(
-        "SELECT id, task, status, kind, created_at FROM topology_runs
+        "SELECT id, task, status, kind, created_at, iteration, finished_at
+         FROM topology_runs
          WHERE workspace_id = $1 ORDER BY created_at DESC LIMIT $2",
         workspace_id.as_uuid(),
         limit,
@@ -286,6 +291,42 @@ pub async fn list_recent(
             status: r.status,
             kind: r.kind,
             created_at: r.created_at,
+            iteration: r.iteration,
+            finished_at: r.finished_at,
+        })
+        .collect())
+}
+
+/// Iterations of a loop, newest first. Uses the partial index
+/// `topology_runs_loop_idx` on `(loop_id, iteration DESC)`.
+pub async fn list_by_loop(
+    pool: &PgPool,
+    workspace_id: WorkspaceId,
+    loop_id: Uuid,
+    limit: i64,
+) -> Result<Vec<TopologyRunSummary>, DbError> {
+    let rows = sqlx::query!(
+        "SELECT id, task, status, kind, created_at, iteration, finished_at
+         FROM topology_runs
+         WHERE workspace_id = $1 AND loop_id = $2
+         ORDER BY iteration DESC NULLS LAST, created_at DESC
+         LIMIT $3",
+        workspace_id.as_uuid(),
+        loop_id,
+        limit,
+    )
+    .fetch_all(pool)
+    .await?;
+    Ok(rows
+        .into_iter()
+        .map(|r| TopologyRunSummary {
+            id: r.id,
+            task: r.task,
+            status: r.status,
+            kind: r.kind,
+            created_at: r.created_at,
+            iteration: r.iteration,
+            finished_at: r.finished_at,
         })
         .collect())
 }