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/NodePanel.tsx

@@ -934,6 +934,32 @@ function PanelBody({
             onChange={(params) => onChange({ ...node, params })}
           />
         ) : null}
+        {hasSource ? (
+          <div className="grid gap-1.5">
+            <Label htmlFor="node-timeout" className={SECTION}>
+              Timeout
+            </Label>
+            <Input
+              id="node-timeout"
+              type="number"
+              min={0}
+              step="any"
+              className="h-8 text-sm"
+              placeholder="30 (default)"
+              value={node.timeout ? String(node.timeout) : ""}
+              onChange={(event) =>
+                onChange({
+                  ...node,
+                  timeout: Number(event.target.value) || null,
+                })
+              }
+            />
+            <p className="text-xs text-muted-foreground">
+              Seconds this code may run before it is stopped. Above 60 the
+              engine may deliver the same work again while it is still running.
+            </p>
+          </div>
+        ) : null}
         {hasSource ? (
           <SharingSection flow={flow} node={node} onShared={onShared} />
         ) : null}