fix(backend): default GET list endpoints to JSON per spec 6.2

Spec 6.2 says: 'Default JSON response wraps the same data in a
{items, total, returned, has_more} envelope so the frontend can
paginate uniformly.' Previously the GET list endpoints defaulted
to NDJSON (consistent with the parse endpoints), which forced
every curl call to set Accept: application/json.

Now NDJSON is strictly opt-in via Accept: application/x-ndjson;
JSON is the default. Parse endpoints keep their existing NDJSON
default (browser uploads).

T8 test 'test_claims_default_accept_returns_ndjson' updated to
assert the new default.

backend/tests/test_api_gets.py

@@ -278,7 +278,12 @@ def test_claims_ndjson_wins_over_json_when_both_accepted(seeded_store):
     assert resp.headers["content-type"].startswith("application/x-ndjson")
 
 
-def test_claims_default_accept_returns_ndjson(seeded_store):
-    """No Accept header → NDJSON (consistent with parse endpoints' default)."""
+def test_claims_default_accept_returns_json(seeded_store):
+    """No Accept header → JSON envelope (per spec 6.2: JSON is the default for
+    list endpoints; NDJSON is the opt-in)."""
     resp = seeded_store.get("/api/claims")
-    assert resp.headers["content-type"].startswith("application/x-ndjson")
+    body = resp.json()
+    assert "items" in body
+    assert "total" in body
+    assert "returned" in body
+    assert "has_more" in body