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/observed_attachments.py

@@ -13,7 +13,7 @@ caller is upgrading ``False/None`` to ``True``.
 from __future__ import annotations
 
 from datetime import datetime, timezone
-from typing import Optional
+from typing import Optional, cast
 
 from sqlalchemy import select
 
@@ -106,4 +106,4 @@ class ObservedAttachmentsMixin(_MixinBase):
                 row.mal_hash_match_at = now
             session.add(row)
             await session.commit()
-            return row.uuid
+            return cast(str, row.uuid)