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>

goodnews/queries.py

@@ -327,6 +327,10 @@ def source_health(conn: sqlite3.Connection) -> list[dict]:
         # Curation quality: of what this source got ACCEPTED, how much was a
         # duplicate of content already served (accepted_total − served = accepted dupes).
         d["accepted_dup_rate"] = round(100 * (accepted - d["served"]) / accepted) if accepted else None
+        # Match the REAL scheduler gate: due = the later of the streak-backoff time
+        # and any retry_after_at rest (UTC strings sort chronologically).
+        due_times = [t for t in (d["next_due_at"], d["retry_after_at"]) if t]
+        d["next_due_at"] = max(due_times) if due_times else None
         out.append(d)
     return out