Keep a flow's runs when the flow goes

A run record is a record of what ran, so deleting a flow no longer sweeps its
history: `_forget_runs` is gone from the flow-delete path, and `DELETE /runs/{id}`
is the only thing that removes a run one at a time. The in-flight guard stays —
that is about work, not history.

`DELETE /runs?flow=` is the counterpart to the list's flow filter and what a
reseed needs, sharing the four statements with the single-run delete and
refusing the same way while a run of that flow is still going.

A run whose flow is gone reads as one: `useFlowInputs` reports the 404 rather
than an empty declaration set, so the run page says "flow deleted", explains it,
and disables Retry, which the route would refuse anyway.

docs/concepts/runs.md

@@ -233,8 +233,9 @@ execution that is not happening this time, so its series is not rewritten
 either. The run it was restored from is recorded instead, and that is where the
 curve is read back from: asking the reusing run for its metrics answers with
 the same points, under its own flow's names. The one way to be left with a
-result and no curve is for that earlier run to have been deleted, which
-deleting its flow does.
+result and no curve is for that earlier run to have been deleted. Deleting a
+flow does not do that — its runs stay — so this only happens when the run
+itself was deleted.
 
 ## Objects that cannot be serialized
 
@@ -346,6 +347,14 @@ the artifacts it made, its metrics and its result. **Retry** submits it again
 as a run of its own, keeping the flow, the inputs, the seed and the sweep it
 belonged to.
 
+A run outlives the flow it ran. Deleting a flow leaves its history standing —
+being the record of what ran is the point of keeping one — so the Runs screen
+still lists it, and opening it says **flow deleted** with Retry disabled,
+because there is nothing left to run it against. Clearing that history is a
+decision of its own: a run at a time on the screen, or
+`DELETE /api/v1/runs?flow=<name>` for a whole flow's worth, which is what a
+reseed wants.
+
 ### Taking it into a dataframe
 
 An analysis wants a table rather than a screen, and there are two it usually
@@ -407,9 +416,8 @@ sweep, or specific runs. It re-reads on its own and whenever a run finishes.
 ### When a run draws nothing
 
 A node restored from the [stage cache](#stage-caching) has its curve read back
-from the run that recorded it. Delete that run, which deleting its flow does,
-and the reusing run is left with a result and an empty curve, and the chart says so
-rather than looking broken.
+from the run that recorded it. Delete that run and the reusing run is left with
+a result and an empty curve, and the chart says so rather than looking broken.
 
 ## What this costs, compared