Flow delete offers to delete its runs, history included

DELETE /runs?flow= removes every Run row of the flow but left its flow_run
rows, so a deleted flow kept appearing in the history. history=true now
drops those too, and the flow delete dialogs (editor and bulk) send it when
"Also delete its runs" is ticked.

docs/code/api.md

@@ -131,7 +131,7 @@ values. You can publish only to declared messages, just as dashboard controls do
 | `GET` | `/runs/{id}/nodes/{node}/outputs` | read what one node returned |
 | `POST` | `/runs/{id}/cancel`, `/runs/{id}/retry` | cancel, or create a retry |
 | `DELETE` | `/runs/{id}` | delete a finished run and its records |
-| `DELETE` | `/runs?flow={name}` | delete a flow's runs; refused while any are active |
+| `DELETE` | `/runs?flow={name}` | delete a flow's runs; refused while any are active. `history=true` also clears its cascades from the history |
 | `GET` | `/runs/{id}/metrics?name=&stride=` | read metric series |
 | `GET` | `/runs/metrics/names` | list metric names across a selection |
 | `GET` | `/runs/series/compare?ids=a,b,c&metric=&x=` | compare curves by step, time, or another metric |