Add per-paper summary verification and cap safeguards

- Per-paper summary batches are fact-checked against abstracts (verify ->
  revise once on major issues -> re-verify), controlled by SEBRIEF_VERIFY_ENABLED
- The paper cap never drops papers already published in this week's digest
  (--force re-runs could otherwise erase them from every digest)
- SEBRIEF_MAX_PAPERS default 40 -> 60 (observed weekly volume ~47)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_config.py

@@ -21,9 +21,11 @@ def test_defaults():
     assert s.run_weekday == 0
     assert s.run_time_kst == "07:00"
     assert s.catchup_window_hours == 72
-    assert s.max_papers == 40
+    assert s.max_papers == 60
     assert s.backfill_days == 7
     assert s.verify_enabled is True
+    assert s.paper_summary_enabled is True
+    assert s.paper_summary_batch_size == 8
     assert s.rss_feeds == []
     assert s.db_path == Path("data") / "sebrief.sqlite3"