Make changing engine-run code superuser-only

Writing node source, installing modules and any flow save that adds,
retypes or repoints a code node (Function, Command), edits a Command
node's settings or changes a node's resources.env now answer 403 for
everyone but a superuser. MCP tools and the assistant reach the same
routes, so they inherit the gate. Node types report runs_code.

The editor shows code read-only with a note, disables a Command node's
settings, leaves code nodes and reusable nodes out of the palette and
paste, and the Modules page offers no install.

frontend/src/routes/_layout/modules.tsx

@@ -6,6 +6,7 @@ import { useState } from "react"
 import { type ApiError, ModulesService } from "@/client"
 import { InfoTip } from "@/components/Flow/SidePanel"
 import { Button } from "@/components/ui/button"
+import useAuth from "@/hooks/useAuth"
 import useCustomToast from "@/hooks/useCustomToast"
 import { handleError } from "@/utils"
 
@@ -32,6 +33,9 @@ function Modules() {
   })
   const queryClient = useQueryClient()
   const { showSuccessToast, showErrorToast } = useCustomToast()
+  // A package runs its own code on install, so installing is a superuser's.
+  const { user } = useAuth()
+  const mayInstall = Boolean(user?.is_superuser)
   // Null until the field is touched, so what is stored shows through until
   // someone actually starts editing it.
   const [draft, setDraft] = useState<string | null>(null)
@@ -84,7 +88,14 @@ function Modules() {
         </p>
       </div>
 
-      {adopted ? null : (
+      {adopted ? null : !mayInstall ? (
+        <p
+          className="text-sm text-muted-foreground"
+          data-testid="modules-locked"
+        >
+          Only a superuser can install packages.
+        </p>
+      ) : (
         <section className="grid gap-3">
           <h2 className={SECTION}>Requirements</h2>
           <textarea