feat(topology): pending-only mutation repo methods with cascade + guards

MazeNET phase 2 step 6. Equips the repo layer with the CRUD the web
editor needs before deploy.

- TopologyNotEditable exception: raised when a pending-only method hits
  a non-pending topology. The intent is "free-form edits stop at deploy;
  the mutator (step 7) takes over for live topologies."
- _assert_pending helper checks status inside the session.
- update_lan / update_topology_decky accept enforce_pending=True for
  pre-deploy callers (existing internal callers default to False so
  behavior is unchanged).
- delete_lan: cascades edges; refuses if any decky has only one edge
  (= this LAN is its home) to prevent orphans.
- delete_topology_decky: cascades edges.
- delete_topology_edge: bare-bones removal.

All four mutators accept expected_version for optimistic concurrency.
Existing tests continue to pass (no behavior change for persist/deploy).

decnet/topology/status.py

@@ -53,6 +53,24 @@ class TopologyStatusError(ValueError):
     """Raised when an illegal topology status transition is attempted."""
 
 
+class TopologyNotEditable(RuntimeError):
+    """Raised when a pending-only mutation hits a non-pending topology.
+
+    Pre-deploy edits (update_lan, delete_lan, update/delete decky,
+    delete_edge) are only legal while the topology is ``pending``.
+    After deploy the mutator's reconciler + topology_mutations table
+    take over.
+    """
+
+    def __init__(self, *, status: str, reason: str = "") -> None:
+        self.status = status
+        self.reason = reason
+        super().__init__(
+            f"topology not editable (status={status!r})"
+            + (f": {reason}" if reason else "")
+        )
+
+
 class VersionConflict(RuntimeError):
     """Raised when a topology write is supplied a stale ``expected_version``.