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/realism/llm/factory.py

@@ -19,7 +19,7 @@ graph cycle-free and the env contract auditable in one place.
 from __future__ import annotations
 
 import os
-from typing import Any
+from typing import Any, cast
 
 from decnet.realism.llm.base import LLMBackend
 
@@ -45,7 +45,7 @@ def get_llm(*, model: str | None = None, **kwargs: Any) -> LLMBackend:
         from decnet.realism.llm.config import get_cached_backend
         cached = get_cached_backend()
         if cached is not None:
-            return cached
+            return cast(LLMBackend, cached)
 
     backend_key = os.environ.get("DECNET_REALISM_LLM", "ollama").lower()