perf(bib): bump CDN download workers 4 → 16

At 4 workers the backfill runs at ~30% of the API rate ceiling —
attachment downloads are the bottleneck. CDN has no published rate
limit and httpx default pool is 100 connections, so 16 leaves plenty
of headroom and should push throughput closer to the 46 items/min API
cap.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/bib/regulations_gov.py

@@ -90,7 +90,7 @@ class Client:
         api_key: str | None = None,
         *,
         sleep: float = 1.3,  # ~46 req/min — under the 50/min burst cap
-        dl_workers: int = 4,  # concurrent attachment download threads
+        dl_workers: int = 16,  # concurrent attachment download threads
         client: httpx.Client | None = None,
     ) -> None:
         key = api_key or os.environ.get("REGULATIONS_GOV_API_KEY")