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.

backend/fluksio/flow/controller.py

@@ -303,6 +303,9 @@ class NodeType:
     # Constructor signatures differ per node type.
     cls: Any
     has_source: bool = False
+    #: Runs code its definition supplies — a source file, or a command among
+    #: its settings — with everything the engine can reach.
+    runs_code: bool = False
     free_params: bool = False
     params_schema: dict[str, Any] = field(default_factory=dict)
     #: Which installed package supplied this type, for the ones that are not
@@ -321,6 +324,7 @@ NODE_TYPES: dict[str, NodeType] = {
         description="Your own Python code, run on every incoming message.",
         cls=Node,
         has_source=True,
+        runs_code=True,
         free_params=True,
     ),
     "mqtt": NodeType(
@@ -399,6 +403,7 @@ NODE_TYPES: dict[str, NodeType] = {
         title="Command",
         description="Run a command in the engine's container and read its output.",
         cls=ExecNode,
+        runs_code=True,
         params_schema=_schema_of(ExecNode),
     ),
     "file": NodeType(
@@ -424,6 +429,7 @@ def node_type_info() -> list[NodeTypeInfo]:
             description=spec.description,
             params_schema=spec.params_schema,
             has_source=spec.has_source,
+            runs_code=spec.runs_code,
             free_params=spec.free_params,
             plugin=spec.plugin,
         )