Run python nodes out of process, with modules of their own

User code no longer execs in the engine. A pool of persistent worker
subprocesses speaks one JSON object per line; the controller installs a
proxy as the node's function, so every execution path funnels through it
and the pipeline is untouched. A crash costs one subprocess, a per-node
timeout is a kill, and cancelling from the canvas is that same kill.

The workers run a venv of the user's own on the data volume, filled from
a pip manifest versioned beside the flows. Applying it retires the
workers and rebuilds, so a package lands without restarting the engine.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017MeiWk3Yq12n2pTvnQWYvt

frontend/src/components/Flow/FlowNode.tsx

@@ -15,12 +15,13 @@ import {
   Radio,
   Shuffle,
   Split,
+  Square,
   Terminal,
   Timer,
 } from "lucide-react"
 import { memo } from "react"
 
-import type { MessageSpec, NodeDef_Input } from "@/client"
+import { FlowsService, type MessageSpec, type NodeDef_Input } from "@/client"
 import { Button } from "@/components/ui/button"
 import {
   Tooltip,
@@ -157,6 +158,32 @@ function FlowNodeComponent({ data, selected }: NodeProps) {
             {typeLabel}
           </span>
         </span>
+        {status === "running" ? (
+          <Tooltip>
+            <TooltipTrigger asChild>
+              <Button
+                variant="ghost"
+                size="icon-sm"
+                className="nodrag nopan -my-1 size-6 shrink-0 text-muted-foreground hover:text-destructive"
+                aria-label="Stop this node"
+                data-testid="node-cancel"
+                onClick={(event) => {
+                  event.stopPropagation()
+                  // Best effort by nature: it may well have finished between
+                  // the render and the click, which is the outcome asked for.
+                  FlowsService.cancelNode({
+                    name: flow,
+                    nodeId: definition.id,
+                  }).catch(() => {})
+                }}
+              >
+                <Square />
+              </Button>
+            </TooltipTrigger>
+            <TooltipContent>Stop this node</TooltipContent>
+          </Tooltip>
+        ) : null}
+
         {status === "error" && onShowLogs ? (
           <Tooltip>
             <TooltipTrigger asChild>