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/sqlmodel_repo/attribution.py

@@ -21,7 +21,7 @@ from __future__ import annotations
 
 import uuid as _uuid
 from datetime import datetime, timezone
-from typing import Any, Optional
+from typing import Any, Optional, cast
 
 from sqlalchemy import func, select
 from sqlmodel import col
@@ -66,7 +66,7 @@ class AttributionMixin(_MixinBase):
             if attacker_row is None:
                 return None
             if attacker_row.identity_id:
-                return attacker_row.identity_id
+                return cast(str, attacker_row.identity_id)
             new_uuid = _uuid.uuid4().hex
             now = datetime.now(timezone.utc)
             session.add(