style: format backend with black

Also drops the session-sliding comment in login_required.

backend/validation.py

@@ -1,4 +1,5 @@
 """Request validation shared by every slice. Errors carry a status, a fix-it message, and the field."""
+
 from flask import request
 
 
@@ -28,7 +29,11 @@ def require_utf8(value: str, field: str) -> None:
     try:
         value.encode("utf-8")
     except UnicodeEncodeError:
-        raise ApiError(400, f"{field} contains unpaired surrogate characters; send valid UTF-8 text", field) from None
+        raise ApiError(
+            400,
+            f"{field} contains unpaired surrogate characters; send valid UTF-8 text",
+            field,
+        ) from None
 
 
 def text(body: dict, field: str, max_len: int) -> str: