refactor(db): split topology.py into a topology/ subpackage

Splits the 694-line topology.py into five submixin files plus a
composing TopologyMixin in topology/__init__.py:

  _core.py       (~225)  topologies CRUD + _assert_pending /
                         _check_and_bump_version concurrency guards
  lans.py        (~115)  LAN CRUD
  deckies.py     (~130)  topology decky CRUD + list_running_topology_deckies
  edges.py        (~80)  edge CRUD + status-event log
  mutations.py   (~165)  live reconciler queue (atomic claim + state writes)

Sibling submixins call self._assert_pending and
self._check_and_bump_version; MRO resolves them onto TopologyCoreMixin
through the composed SQLModelRepository class.

decnet/web/db/sqlmodel_repo/topology/__init__.py

@@ -0,0 +1,32 @@
+"""MazeNET topology repository methods.
+
+The full domain spans ~700 lines of methods across topologies, LANs,
+deckies, edges, the status-event log, and the live reconciler mutation
+queue.  Each concern lives in its own submixin; ``TopologyMixin``
+composes them.
+
+The optimistic-locking helpers (``_assert_pending``,
+``_check_and_bump_version``) live on ``TopologyCoreMixin`` and are
+reached from sibling submixins via ``self.`` — Python's MRO resolves
+them to the core mixin no matter which submixin holds the caller.
+"""
+from __future__ import annotations
+
+from decnet.web.db.sqlmodel_repo.topology._core import TopologyCoreMixin
+from decnet.web.db.sqlmodel_repo.topology.deckies import TopologyDeckiesMixin
+from decnet.web.db.sqlmodel_repo.topology.edges import TopologyEdgesMixin
+from decnet.web.db.sqlmodel_repo.topology.lans import LansMixin
+from decnet.web.db.sqlmodel_repo.topology.mutations import TopologyMutationsMixin
+
+
+class TopologyMixin(
+    TopologyDeckiesMixin,
+    TopologyEdgesMixin,
+    LansMixin,
+    TopologyMutationsMixin,
+    TopologyCoreMixin,
+):
+    """Composed topology mixin — see submixins for the actual methods."""
+
+
+__all__ = ["TopologyMixin"]