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/web/db/factory.py

@@ -6,7 +6,7 @@ Repository factory — selects a :class:`BaseRepository` implementation based on
 from __future__ import annotations
 
 import os
-from typing import Any
+from typing import Any, cast
 
 from decnet.web.db.repository import BaseRepository
 
@@ -32,4 +32,4 @@ def get_repository(**kwargs: Any) -> BaseRepository:
         raise ValueError(f"Unsupported database type: {db_type}")
 
     from decnet.telemetry import wrap_repository
-    return wrap_repository(repo)
+    return cast(BaseRepository, wrap_repository(repo))