test(reader): the service tests hold each rule their mutant broke

A reorder's left-out item, a playlisted video beside one not asked about, another user's tag, and
the user's own channel link committed before the auto-tagger reads the table.

backend/tests/test_feed_cursor.py

@@ -1,7 +1,7 @@
 """Feed cursor endpoint: a malformed ?cursor= must be a clean HTTP 400, not a 500.
 
-Locks the refactor that split cursor handling across modules — feedquery.decode_cursor raises a
-plain ValueError and routes/feed.get_feed translates it to 400 — so a future regression (a dropped
+Cursor handling spans modules — feedquery.decode_cursor raises a plain ValueError, feed_page turns
+it into BadCursor, and routes/feed.get_feed translates that to 400 — so a regression (a dropped
 try/except, or a new decode_cursor caller without its own catch) turns the 400 back into a 500 and
 fails right here. DB lane (`db` fixture) — skipped without Postgres.
 """