c2-core: Archive "Load more" skipped 150 of every 200 calls

/calls/search scans a 200-row window and returns 50, but the next cursor
was always the last SCANNED row — so with an unfiltered list each page
jumped past the 150 matches it had already read and not shown. Resume
after the last RETURNED row when matches overflow the page; keep the
last-scanned cursor only when the page holds every match (the sparse-
filter case that cursor exists for). Same fix for /calls/eval-queue.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-c2-core/tests/test_calls_parse_ts.py

@@ -8,7 +8,7 @@ from datetime import datetime, timezone
 import pytest
 from fastapi import HTTPException
 
-from app.routers.calls import _parse_ts
+from app.routers.calls import _next_cursor, _parse_ts
 
 
 def test_empty_is_none():
@@ -33,3 +33,27 @@ def test_garbage_is_400():
     with pytest.raises(HTTPException) as exc:
         _parse_ts("yesterday", "date_from")
     assert exc.value.status_code == 400
+
+
+# ── _next_cursor ──────────────────────────────────────────────────────────
+
+
+def _rows(n):
+    return [{"started_at": datetime(2026, 9, 20, 12, i // 60, i % 60, tzinfo=timezone.utc)} for i in range(n)]
+
+
+def test_cursor_resumes_after_last_returned_row_when_matches_overflow():
+    rows = _rows(200)
+    page = rows[:50]
+    assert _next_cursor(rows, rows, page, 200) == page[-1]["started_at"].isoformat()
+
+
+def test_cursor_resumes_after_window_when_page_holds_every_match():
+    rows = _rows(200)
+    matches = rows[:3]
+    assert _next_cursor(rows, matches, matches, 200) == rows[-1]["started_at"].isoformat()
+
+
+def test_short_window_is_the_end():
+    rows = _rows(20)
+    assert _next_cursor(rows, rows[:5], rows[:5], 200) is None