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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CL9zvnnvcp1mvA8o7impxk

backend/fluksio/api/routes/flows.py

@@ -16,7 +16,6 @@ from fastapi import (
 from fastapi.concurrency import run_in_threadpool
 from jwt.exceptions import InvalidTokenError
 from pydantic import BaseModel
-from sqlalchemy import delete
 from sqlmodel import Session, col, select
 
 from fluksio.api.deps import (
@@ -59,7 +58,7 @@ from fluksio.flow.store import (
     LibNotFound,
     StaleVersion,
 )
-from fluksio.models import Flavor, Message, Run, RunArtifact, RunMetric, RunNode
+from fluksio.models import Flavor, Message, Run
 
 router = APIRouter(
     prefix="/flows", tags=["flows"], dependencies=[Depends(get_current_user)]
@@ -242,25 +241,6 @@ def _audit(action: str, flow: str, user: CurrentUser) -> None:
     )
 
 
-def _forget_runs(session: Session, flow: str) -> None:
-    """A deleted flow's runs, and everything hanging off them.
-
-    Here rather than in ``FlowController.forget_flow`` because renaming a flow
-    calls that too, and a rename must keep its experiment history.
-
-    Only the run tables: ``flow_run``, ``metric_minute`` and ``engine_event``
-    are the observability rollups, deliberately kept as a record of what ran
-    and already pruned at OBS_RETENTION_DAYS.
-    """
-    # A subquery, not a materialised list of ids: a demo can hold thousands.
-    runs = select(col(Run.id)).where(col(Run.flow) == flow)
-    session.execute(delete(RunNode).where(col(RunNode.run_id).in_(runs)))
-    session.execute(delete(RunMetric).where(col(RunMetric.run_id).in_(runs)))
-    session.execute(delete(RunArtifact).where(col(RunArtifact.run_id).in_(runs)))
-    session.execute(delete(Run).where(col(Run.flow) == flow))
-    session.commit()
-
-
 def _source_ref(definition: FlowDef, node_id: str) -> str | None:
     """The library source this node runs, if it is a shared one."""
     node = next((n for n in definition.nodes if n.id == node_id), None)
@@ -483,7 +463,7 @@ async def discard_draft(name: str, controller: FlowControllerDep) -> Any:
 async def delete_flow(
     name: str, controller: FlowControllerDep, user: CurrentUser, session: SessionDep
 ) -> Any:
-    """Delete a flow and everything in it."""
+    """Delete a flow and everything in it, except the record of what it ran."""
     live = session.exec(
         select(Run.id).where(
             col(Run.flow) == name, col(Run.status).in_(("running", "queued"))
@@ -503,8 +483,10 @@ async def delete_flow(
         raise HTTPException(status_code=404, detail=f"No flow named '{name}'")
     _audit("deleted", name, user)
     # Its files are gone; its values and queued work would otherwise linger.
+    # Its runs stay: a run record is a record of what ran, and surviving the
+    # flow it belonged to is the point of keeping one. `DELETE /runs?flow=`
+    # is what clears them.
     await run_in_threadpool(controller.forget_flow, name)
-    await run_in_threadpool(_forget_runs, session, name)
     await controller.reload_flow(name)
     return Message(message=f"Deleted flow '{name}'")