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/templates/mongodb/server.py

@@ -147,7 +147,7 @@ _MONGO_SET_NAME = os.environ.get("MONGO_REPL_SET", "")  # empty = standalone
 
 def _new_objectid() -> bytes:
     """12-byte BSON ObjectId — fresh per call."""
-    return _seed.fresh_bytes(12)
+    return cast(bytes, _seed.fresh_bytes(12))
 
 # Minimal BSON helpers
 def _bson_str(key: str, val: str) -> bytes: