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/src/cyclone/api.py

@@ -143,21 +143,19 @@ def _client_wants_json(request: Request) -> bool:
 
 
 def _wants_ndjson(request: Request) -> bool:
-    """Content negotiation for list endpoints: NDJSON wins ties, NDJSON is the
-    default when no Accept header is set (consistent with parse endpoints).
+    """Content negotiation for list endpoints: NDJSON is an opt-in, JSON is the
+    default (per spec 6.2: "Default JSON response wraps the same data in a
+    {items, total, returned, has_more} envelope so the frontend can paginate
+    uniformly").
 
     Used by the GET list routes (/api/batches, /api/claims, /api/remittances,
-    /api/providers, /api/activity). When both ``application/x-ndjson`` and
-    ``application/json`` appear in Accept, NDJSON wins. When only JSON is
-    asked for, JSON wins. When neither is asked for (e.g. browser default
-    ``*/*`` or an empty Accept), NDJSON wins.
+    /api/providers, /api/activity). NDJSON is returned only when the client
+    explicitly sends ``Accept: application/x-ndjson`` (with or without
+    ``application/json``). Bare ``*/*``, an empty Accept, or an explicit
+    ``Accept: application/json`` all return the JSON envelope.
     """
     accept = request.headers.get("accept", "")
-    if "application/x-ndjson" in accept:
-        return True
-    if "application/json" in accept:
-        return False
-    return True
+    return "application/x-ndjson" in accept
 
 
 def _ndjson_stream_list(