fix(topology): delete topology_mutations in delete-cascade

delete_topology_cascade manually deletes status_events, edges, deckies
and lans but overlooked topology_mutations, so deleting any topology
that ever had a mutation enqueued (i.e. edits while active|degraded)
failed with an FK IntegrityError. Add the missing DELETE and extend
the cascade test to seed a mutation row.

decnet/web/db/sqlmodel_repo.py

@@ -1111,6 +1111,10 @@ class SQLModelRepository(BaseRepository):
                 text("DELETE FROM lans WHERE topology_id = :t"),
                 params,
             )
+            await session.execute(
+                text("DELETE FROM topology_mutations WHERE topology_id = :t"),
+                params,
+            )
             result = await session.execute(
                 select(Topology).where(Topology.id == topology_id)
             )