feat(api): phase 3 step 1 — topology request/response models + router skeleton

Add Pydantic DTOs in decnet/web/db/models.py covering every phase-3
endpoint shape: TopologyGenerateRequest, TopologySummary/Detail, child
create/update requests, MutationEnqueueRequest (Literal op guard),
MutationRow with JSON-payload decoder, validation/version/not-editable
error envelopes, and the three catalog responses.

Create decnet/web/router/topology/ as an import-safe package exporting
topology_router (prefix /topologies) — sub-routers land step-by-step in
subsequent commits. Mount under the main api router alongside swarm_mgmt.

tests/api/topology/test_models.py pins repo-dict ↔ DTO parity so future
repo-row drift breaks the contract test before the endpoints.

decnet/web/router/__init__.py

@@ -24,6 +24,7 @@ from .artifacts.api_get_artifact import router as artifacts_router
 from .swarm_updates import swarm_updates_router
 from .swarm_mgmt import swarm_mgmt_router
 from .system import system_router
+from .topology import topology_router
 
 api_router = APIRouter(
     # Every route under /api/v1 is auth-guarded (either by an explicit
@@ -83,3 +84,6 @@ api_router.include_router(swarm_mgmt_router)
 
 # System info (deployment-mode auto-detection, etc.)
 api_router.include_router(system_router)
+
+# MazeNET Topologies (nested topology CRUD + mutation queue)
+api_router.include_router(topology_router)

decnet/web/router/topology/__init__.py

@@ -0,0 +1,18 @@
+"""MazeNET topology REST endpoints (phase 3).
+
+Thin FastAPI layer over the phase-2 topology machinery:
+generate/validate/deploy/teardown, pending-only child CRUD, and the
+live-mutation queue for active|degraded topologies.
+
+Mounted at ``/api/v1/topologies`` by the main api router.  Sub-routers
+live one-per-file and are aggregated here.
+"""
+from fastapi import APIRouter
+
+topology_router = APIRouter(prefix="/topologies", tags=["topologies"])
+
+# Sub-routers land in later steps; this skeleton keeps the package
+# import-safe so the main api router can mount it immediately.
+
+
+__all__ = ["topology_router"]