feat(api): phase 3 step 2 — topology read endpoints (list/get/status/catalog)

GET /api/v1/topologies — paginated list with status filter. Extends
repo.list_topologies() to accept limit/offset and adds count_topologies()
for the total envelope field.

GET /api/v1/topologies/{id} — hydrated TopologyDetail; 404 if missing.
GET /api/v1/topologies/{id}/status-events — audit trail, limit-capped.

Catalog helpers for the phase-4 canvas UI:
* GET /topologies/services — full service catalog.
* GET /topologies/next-subnet?base=172.20 — wraps SubnetAllocator against
  reserved_subnets across non-torn-down topologies.
* GET /topologies/{id}/lans/{lan_id}/next-ip — IPAllocator pre-seeded
  with existing decky IPs in that LAN.

All read routes are viewer-or-admin. Sub-routers are included in an
order that keeps literal catalog paths (/services, /next-subnet) from
being shadowed by the /{topology_id} trie branch.

decnet/web/db/sqlmodel_repo.py

@@ -949,11 +949,18 @@ class SQLModelRepository(BaseRepository):
             return self._deserialize_json_fields(d, ("config_snapshot",))
 
     async def list_topologies(
-        self, status: Optional[str] = None
+        self,
+        status: Optional[str] = None,
+        limit: Optional[int] = None,
+        offset: Optional[int] = None,
     ) -> list[dict[str, Any]]:
         statement = select(Topology).order_by(desc(Topology.created_at))
         if status:
             statement = statement.where(Topology.status == status)
+        if offset is not None:
+            statement = statement.offset(offset)
+        if limit is not None:
+            statement = statement.limit(limit)
         async with self._session() as session:
             result = await session.execute(statement)
             return [
@@ -963,6 +970,15 @@ class SQLModelRepository(BaseRepository):
                 for r in result.scalars().all()
             ]
 
+    async def count_topologies(self, status: Optional[str] = None) -> int:
+        from sqlalchemy import func
+        statement = select(func.count(Topology.id))
+        if status:
+            statement = statement.where(Topology.status == status)
+        async with self._session() as session:
+            result = await session.execute(statement)
+            return int(result.scalar_one() or 0)
+
     async def update_topology_status(
         self,
         topology_id: str,