fix(runs): stamp completed_at on cancel/terminal SetState transitions

CancelRun goes through Runner.Transition → Runs.SetState, which was a
bare UPDATE state=? with no completed_at write. The host-page
runDuration helper treats nil CompletedAt as "still running", so a
cancelled run kept ticking forever. MarkCompleted / MarkFailed /
MarkDispatchFailed already stamp completed_at; SetState now does the
same for any terminal target state, using COALESCE so we never
clobber an already-set timestamp.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/store/runs.go

@@ -45,6 +45,13 @@ func (r *Runs) CreateWithProfile(ctx context.Context, hostID int64, tokenHash st
 }
 
 func (r *Runs) SetState(ctx context.Context, runID int64, state model.RunState) error {
+	if state.IsTerminal() {
+		_, err := r.DB.ExecContext(ctx, `
+			UPDATE runs SET state = ?, completed_at = COALESCE(completed_at, ?)
+			WHERE id = ?
+		`, string(state), time.Now().UTC(), runID)
+		return err
+	}
 	_, err := r.DB.ExecContext(ctx, `UPDATE runs SET state = ? WHERE id = ?`, string(state), runID)
 	return err
 }