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/code/api.md

@@ -46,7 +46,7 @@ Agents authenticate differently; see [Agents over MCP](agents.md).
 | `POST` | `/flows/{name}/publish` | put the draft live |
 | `POST` | `/flows/{name}/discard-draft` | throw the draft away |
 | `POST` | `/flows/{name}/rename` | rename it |
-| `DELETE` | `/flows/{name}` | delete it |
+| `DELETE` | `/flows/{name}` | delete it. Its runs stay — a run is the record of what ran, and only `DELETE /runs` clears one. Refused with 409 while a run of it is `running` or `queued` |
 | `GET` | `/flows/node-types` | every node type and its parameter schema |
 | `GET` | `/flows/graph` | every flow as one graph — what Home draws |
 
@@ -104,12 +104,14 @@ published to. Flows own the namespace; everything else is a client of it.
 | `POST` | `/runs/flows/{name}` | queue one run — `{"params": {...}, "seed": 7, "draft": false, "no_cache": false}`. `"cause"` says where it came from — `api` (the default), `cli` or `sdk` |
 | `POST` | `/runs/flows/{name}/sweep` | queue many, sharing a `group_id` |
 | `GET` | `/runs` | the queryable history: `?flow=`, `?status=`, `?group=`, `?digest=`, `?since=`, `?before=`, `?limit=`, `?offset=` |
+| `DELETE` | `/runs?flow=` | every run of one flow, and everything hanging off them. `flow` is required. Refused with 409 while one of them is still going |
 | `GET` | `/runs/overview` | one row per flow that has runs, with how many are running or queued |
 | `GET` | `/runs/export/metrics?…&name=&stride=&format=` | every selected run's series as one long table: `run, name, step, ts, value` |
 | `GET` | `/runs/export/runs?…&params=&metrics=&format=` | one row per run: its inputs as columns, its final numbers, its status and provenance |
 | `GET` | `/runs/metrics/names?…` | every metric name the selected runs recorded, distinct; takes the export's own filters |
 | `GET` | `/runs/{id}` | one run in full: params, result, per-node record, artifacts |
 | `POST` | `/runs/{id}/cancel` | stop it |
+| `DELETE` | `/runs/{id}` | forget it, with its nodes, series and artifact rows. Cancel a live one first |
 | `POST` | `/runs/{id}/retry` | run the same thing again, as a new run naming this one |
 | `GET` | `/runs/{id}/metrics?name=&stride=` | one metric's series, in step order; every series of the run without `name` |
 | `GET` | `/runs/series/compare?ids=a,b,c&metric=&x=` | that metric across several runs. `x` is what to plot against: nothing or `step`, `time` (seconds since each run's own first reading), or another metric's name, joined on the step the two share |