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/correlation/attribution/aggregate.py

@@ -25,7 +25,7 @@ from __future__ import annotations
 
 from collections import Counter
 from dataclasses import dataclass
-from typing import Any, Sequence
+from typing import Any, Sequence, cast
 
 from decnet.correlation.attribution import _thresholds as _T
 
@@ -250,7 +250,7 @@ def _coef_of_variation(values: Sequence[float], mean: float) -> float:
     stdev = variance ** 0.5
     if mean == 0:
         return 0.0 if stdev == 0 else 1e9
-    return stdev / abs(mean)
+    return cast(float, stdev / abs(mean))
 
 
 def _safe_float(value: Any) -> float: