chore(types): enable warn_return_any and cast all no-any-return sites

Turn on mypy warn_return_any (pyproject) and resolve the 84 resulting
[no-any-return] errors across 43 files with typing.cast() at the return
sites — runtime no-ops that make the declared return type explicit where a
dependency (SQLAlchemy scalar/first/one, httpx .json(), subprocess, docker
SDK) hands back Any. No behavior change: no DTO/table field types altered, no
validation/coercion calls added, every cast reflects the true runtime type.

Locks in return-type strictness so the class of bug where a function silently
widens to Any can't regress. mypy decnet/ clean; adversarially verified
behavior-preserving (84 casts 1:1 with prior returns).

Bump tornado 6.5.5 -> 6.5.7 (CVE-2026-49854, transitive via snakeviz).

decnet/mutator/ops.py

@@ -25,7 +25,7 @@ Design notes
 from __future__ import annotations
 
 import json
-from typing import Any, Awaitable, Callable, Optional
+from typing import Any, Awaitable, Callable, Optional, cast
 
 from decnet.logging import get_logger
 from decnet.topology.allocator import IPAllocator, reserved_subnets, SubnetAllocator
@@ -301,7 +301,7 @@ async def _live_topology_or_none(
             topology_id,
         )
         return None
-    return topology
+    return cast(dict[str, Any], topology)
 
 
 async def _rerender_compose(repo: Any, topology_id: str) -> None: