feat: target workflow runs by tag selector

server/internal/api/workflows.go

@@ -343,6 +343,10 @@ func deleteWorkflow(c *gin.Context) {
 func runWorkflow(c *gin.Context) {
 	runID, err := services.TriggerWorkflow(auth.InstanceID(c), c.Param("id"), actorFromCtx(c))
 	if err != nil {
+		if errors.Is(err, services.ErrNoTargets) {
+			c.JSON(http.StatusBadRequest, gin.H{"error": "this workflow matches no servers"})
+			return
+		}
 		c.JSON(http.StatusServiceUnavailable, gin.H{"error": err.Error()})
 		return
 	}