Second-round /code-review high follow-ups:
- test_feed_cursor.py: GET /api/feed?cursor=<garbage> must return HTTP 400
(not 500) — locks the cursor-error path the refactor split across modules
(feedquery.decode_cursor raises ValueError, get_feed translates to 400).
Plus a no-cursor 200 sanity check. DB lane.
- get_feed: `raise HTTPException(...) from None` so the expected bad-input
ValueError isn't chained as __context__.
DB-lane suite 289 passed / 2 skipped; ruff clean.
