Retire a shared node in use with ?detach=true

DELETE /flows/library/{name} still answers 409 while nodes use the source.
With detach=true each of them keeps a copy of the code, in the published
flow and the draft, the source is deleted and the running users rebuild.

backend/fluksio/api/routes/flows.py

@@ -336,8 +336,23 @@ def read_library(controller: FlowControllerDep) -> Any:
 
 
 @router.delete("/library/{lib_name}", response_model=Message)
-async def delete_shared_node(lib_name: str, controller: FlowControllerDep) -> Any:
-    """Remove a shared source, as long as no flow still runs it."""
+async def delete_shared_node(
+    lib_name: str, controller: FlowControllerDep, detach: bool = False
+) -> Any:
+    """Remove a shared source, as long as no flow still runs it.
+
+    With ``detach``, every node using it keeps a copy of the code as its own
+    first, so the source can be retired while flows still use it.
+    """
+    if detach:
+        running = await run_in_threadpool(controller.store.usages, lib_name, False)
+        try:
+            await run_in_threadpool(controller.store.detach_lib, lib_name)
+        except LibNotFound:
+            raise HTTPException(status_code=404, detail=f"No shared node '{lib_name}'")
+        for flow in sorted({usage.split(".", 1)[0] for usage in running}):
+            await controller.reload_flow(flow)
+        return Message(message=f"Deleted shared node '{lib_name}'")
     used_by = controller.store.usages(lib_name)
     if used_by:
         raise HTTPException(