feat(mutator,web): live topology mutation pipeline backend (DEBT-030)

Wire the mutator and web API into the service bus so live-topology
edits flow sub-second from enqueue to UI:

- Mutator publishes every state transition on the bus (mutation.applying
  /applied/failed + topology.status). Fire-and-forget; DB stays source
  of truth.
- Mutator watch loop subscribes to topology.*.mutation.enqueued and
  wakes early via asyncio.Event — the 10s poll becomes a fallback
  heartbeat, not the primary dispatch trigger.
- POST /topologies/{id}/mutations publishes mutation.enqueued after
  the DB write succeeds.
- New GET /topologies/{id}/events SSE route: snapshot on connect
  (status + in-flight mutations), live forwards topology.{id}.>
  bus events, 15s keepalive. ?token= auth mirrors /stream.
- New decnet/bus/app.py — process-wide lazy bus singleton for the
  API, closed cleanly on lifespan shutdown.

decnet/web/router/topology/__init__.py

@@ -16,6 +16,7 @@ from .api_decky_crud import router as _decky_router
 from .api_delete_topology import router as _delete_router
 from .api_deploy_topology import router as _deploy_router
 from .api_edge_crud import router as _edge_router
+from .api_events import router as _events_router
 from .api_get_topology import router as _get_router
 from .api_lan_crud import router as _lan_router
 from .api_list_topologies import router as _list_router
@@ -40,6 +41,7 @@ topology_router.include_router(_lan_router)
 topology_router.include_router(_decky_router)
 topology_router.include_router(_edge_router)
 topology_router.include_router(_mutations_router)
+topology_router.include_router(_events_router)
 topology_router.include_router(_get_router)