source_health: next_due_at = later of streak-backoff and retry_after_at

Per Codex: the Next poll column computed only the streak-backoff time, so a
rate-limited source could show an earlier Next poll than the real gate (which
also requires retry_after_at <= now). Take the later of the two in the Python
post-process so the admin table agrees with due_source_rows.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_retry_after.py

@@ -63,3 +63,14 @@ def test_non_429_failure_still_increments_streak(monkeypatch):
     res = feeds.poll_source(c, src)
     assert res["status"] == "failed"
     assert c.execute("SELECT consecutive_failures FROM sources WHERE id=1").fetchone()[0] == 1
+
+
+def test_source_health_next_due_uses_later_of_backoff_and_retry_after():
+    from goodnews import queries
+    c = connect(":memory:"); init_db(c); _src(c)
+    # a recent attempt (streak due ~soon) but a far-future retry_after_at
+    c.execute("INSERT INTO ingest_runs (source_id, finished_at, status) VALUES (1, datetime('now'), 'rate_limited')")
+    c.execute("UPDATE sources SET retry_after_at = '2099-01-01 00:00:00' WHERE id = 1")
+    c.commit()
+    s = next(x for x in queries.source_health(c) if x["id"] == 1)
+    assert s["next_due_at"] == "2099-01-01 00:00:00"  # agrees with the real gate, not the streak time