fix(feed): a hand-made relevance cursor is refused, not a crash

The page halves compare the cursor's timestamp with the window's in Python;
a null or naive one raised there, past the route's 400. decode_cursor now
refuses a null non-nullable key and a naive timestamp.

backend/app/feedquery.py

@@ -653,18 +653,30 @@ def encode_cursor(keys: list[dict], row, window: datetime | None = None) -> str:
     return base64.urlsafe_b64encode(payload.encode()).decode()
 
 
+def _cursor_dt(v):
+    """A cursor's timestamp: aware or refused. The server writes only aware ones (timestamptz), and
+    windowed_relevance_rows compares them in Python, where a naive one beside an aware one raises — a
+    500 for a hand-made cursor instead of the 400 (E31 review 2)."""
+    if v is None:
+        return None
+    dt = datetime.fromisoformat(v)
+    if dt.tzinfo is None:
+        raise ValueError("naive cursor timestamp")
+    return dt
+
+
 def decode_cursor(keys: list[dict], cursor: str) -> tuple[list, str, datetime | None]:
     try:
         payload = json.loads(base64.urlsafe_b64decode(cursor.encode()))
         raw = payload["k"]
         if len(raw) != len(keys):
             raise ValueError("key arity mismatch")
-        vals = [
-            datetime.fromisoformat(v) if (v is not None and k["kind"] == "dt") else v
-            for k, v in zip(keys, raw)
-        ]
+        vals = [_cursor_dt(v) if k["kind"] == "dt" else v for k, v in zip(keys, raw)]
+        for k, v in zip(keys, vals):
+            if v is None and not k["nullable"]:
+                raise ValueError("null value for a non-nullable key")
         window = payload.get("w")
-        return vals, payload["id"], datetime.fromisoformat(window) if window else None
+        return vals, payload["id"], _cursor_dt(window) if window else None
     except (binascii.Error, json.JSONDecodeError, KeyError, ValueError, TypeError) as exc:
         # Framework-free: the route handler translates this into an HTTP 400 so the
         # read-model stays web-layer-agnostic (see routes/feed.get_feed).