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/tests/test_feed_relevance_window.py

@@ -5,6 +5,8 @@ The ranking now covers the newest RANK_WINDOW matches and the rest follow newest
 rides the cursor, so a video ingested mid-scroll cannot push a row behind the cursor and repeat it.
 """
 
+import base64
+import json
 from datetime import datetime, timezone
 
 import pytest
@@ -112,3 +114,20 @@ def test_cursor_walk_matches_the_unpaged_order(db, as_user, monkeypatch, window,
         page, cursor = _ids(client, cursor, limit=limit)
         walked.extend(page)
     assert walked == full
+
+
+@pytest.mark.parametrize(
+    "payload",
+    [
+        {"k": [5, None], "id": "m1", "w": "2024-01-01T00:00:00+00:00"},  # null published_at
+        {"k": [5, "2024-01-03T00:00:00+00:00"], "id": "m1", "w": "2024-01-01T00:00:00"},  # naive w
+        {"k": [5, "2024-01-03T00:00:00"], "id": "m1", "w": "2024-01-01T00:00:00+00:00"},  # naive k
+    ],
+)
+def test_a_hand_made_relevance_cursor_is_refused_not_a_crash(db, as_user, payload):
+    # The page halves compare the cursor's timestamp with the window's in Python: a null or a naive
+    # one raised there, past the route's 400 — a 500 (E31 review 2).
+    client = as_user(_seed(db))
+    cursor = base64.urlsafe_b64encode(json.dumps(payload).encode()).decode()
+    resp = client.get(f"/api/feed?q=foo&sort=relevance&limit=2&cursor={cursor}")
+    assert resp.status_code == 400, resp.text