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/engine/deployer.py

@@ -9,6 +9,7 @@ import shutil
 import subprocess  # nosec B404
 import time
 from pathlib import Path
+from typing import cast
 
 import anyio
 import docker
@@ -853,7 +854,7 @@ async def _resolve_swarm_host(repo, host_uuid: str) -> dict:
         raise ValueError(
             f"topology pinned to unknown swarm host {host_uuid!r}"
         )
-    return host
+    return cast(dict, host)
 
 
 async def _deploy_on_agent(repo, topology_id: str, hydrated: dict) -> None: