test(feed): cover malformed-cursor 400 + from-None on the re-raise

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.

backend/app/routes/feed.py

@@ -104,7 +104,8 @@ def get_feed(
         try:
             vals, cursor_id = decode_cursor(keys, cursor)
         except ValueError:
-            raise HTTPException(status_code=400, detail="Invalid feed cursor")
+            # `from None`: the ValueError is an expected bad-input signal, not an error to chain.
+            raise HTTPException(status_code=400, detail="Invalid feed cursor") from None
         for k, v in zip(keys, vals):
             k["value"] = v
         query = query.where(keyset_predicate(keys, cursor_id))

backend/tests/test_feed_cursor.py

@@ -0,0 +1,45 @@
+"""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
+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.
+"""
+from fastapi.testclient import TestClient
+
+import app.state as state
+from app.auth import current_user
+from app.db import get_db
+from app.main import app
+from app.models import User
+
+
+def _get(db, user, monkeypatch, path):
+    monkeypatch.setattr(state, "_configured_cache", True)  # the request gate 503s an unconfigured app
+    app.dependency_overrides[get_db] = lambda: db
+    app.dependency_overrides[current_user] = lambda: user
+    try:
+        client = TestClient(app)  # no context manager: skip lifespan (scheduler/HLS sweep)
+        return client.get(path)
+    finally:
+        app.dependency_overrides.clear()
+
+
+def test_malformed_feed_cursor_returns_400(db, monkeypatch):
+    user = User(email="feedcursor@example.com")
+    db.add(user)
+    db.commit()
+    resp = _get(db, user, monkeypatch, "/api/feed?cursor=not-a-real-cursor")
+    assert resp.status_code == 400
+    assert resp.json()["detail"] == "Invalid feed cursor"
+
+
+def test_feed_without_cursor_is_200(db, monkeypatch):
+    # Sanity: the same endpoint without a cursor is a normal 200 (proves the 400 above is the
+    # cursor path, not a globally-broken request). A fresh user has no subscriptions → empty feed.
+    user = User(email="feednocursor@example.com")
+    db.add(user)
+    db.commit()
+    resp = _get(db, user, monkeypatch, "/api/feed")
+    assert resp.status_code == 200
+    assert resp.json()["items"] == []