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/api.py

@@ -8,7 +8,7 @@ import os
 import traceback
 import uuid
 from contextlib import asynccontextmanager
-from typing import Any, AsyncGenerator, Optional
+from typing import Any, AsyncGenerator, Optional, cast
 
 from fastapi import FastAPI, Request, status
 from fastapi.exceptions import RequestValidationError
@@ -306,7 +306,7 @@ class _ContentTypeMiddleware(BaseHTTPMiddleware):
                         status_code=415,
                         media_type="text/plain",
                     )
-        return await call_next(request)
+        return cast(StarletteResponse, await call_next(request))
 
 
 app.add_middleware(_ContentTypeMiddleware)