crawler: --no-cover, --concurrency, drop cross-host sleep + batch-50 Step 1 done

Three crawler ergonomics for batch operations:

--no-cover  Skip cover image download. For scan-only modes (license/meta
            scrape) this drops ~1.3s/project and avoids slow-CDN hangs.

--concurrency N  ThreadPoolExecutor wrapping the per-project loop. Default
                 1 = serial (current behavior). Anonymous endpoints tolerate
                 5+ comfortably; output uses a print lock for readable
                 interleaved progress. fetch_cover plumbs through crawl_one.

Drop cross-host sleep #1: in crawl_one between detail HTML (oshwhub.com)
and cover image (image.lceda.cn). Different hosts — sleep was unnecessary.
Saves ~1s/project. Sleep #2 (post-cover, before next iteration) stays — it
gates the next oshwhub.com hit.

download_to gains max_seconds wall budget (default 60s, cover uses 15s).
Defends against pathologically slow CDN connections — observed 10 KB/s
on image.lceda.cn for one project, would have hung 6+ min on a 3.6 MB
cover otherwise. httpx default timeout resets per chunk, so streaming
downloads need an external wall-clock guard.

batch-50 Step 1 (license/meta scrape) shipped:
  50/50 candidates have metadata.json + license recorded
  License distribution: GPL 3.0 32, Public Domain 6, NC variants 8,
                        CERN-OHL 1, MIT 1, CC BY 3.0 1
  Forge-friendly (non-NC): 41/50 (82%)
  Declared attachments: 180 files / 2.36 GB (median 18 MB/proj, max 304 MB)
  Walltime: 3min 26s for 28 projects at concurrency=5 (server-side
            HTML render bound, not sleep-bound)

One orphan partial cover (a670e60a...) cleaned up — leftover from the
first aborted run before the timeout fix landed.

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

data/raw/oshwhub/4f06b185aae542eb94497acef0a9144b/_urls.json

@@ -0,0 +1,26 @@
+{
+  "detail_url": "https://oshwhub.com/Kirito/usb3-0-wang-ka-rtl8153b",
+  "cover_url": "https://image.lceda.cn/pullimage/k50OonL2mnyuvO11hkTcXaehnWoD76WFjPljTOVi.jpeg",
+  "attachments": [
+    {
+      "name": "VID_20211116_190318.mp4",
+      "url": "https://image.lceda.cn/attachments/2021/11/ZLbCJftJSo7wYs9T1r1tPkLPAQJi4ujAmBDZ9nyh.mp4",
+      "original_id": "c8cec70a7edf4bcb8b64838c11c3e28d"
+    },
+    {
+      "name": "RTL网卡驱动（Windows+Linux系统）.rar",
+      "url": "https://image.lceda.cn/attachments/2021/11/yhMp3UDkmZSQ0KF8xS9wLhyoswY5XGlGpAw3UvYg.rar",
+      "original_id": "3e1d32f8853040849f613b7ed0e81cb6"
+    },
+    {
+      "name": "RTL8153B-VB-CG_Datasheet_1.4.pdf",
+      "url": "https://image.lceda.cn/attachments/2021/11/sA8zbB0lUgY6d2eRMxYs4OhUakikILZNCmPnblHH.pdf",
+      "original_id": "0100985d34d74d6285098b4413931a1a"
+    },
+    {
+      "name": "realtek_usb_lan_tool.zip",
+      "url": "https://image.lceda.cn/attachments/2021/12/MQOAICWSE94IAT1qcKq7pYlK5H1rOSNFyekyIXqm.zip",
+      "original_id": "aa77c2f61cc34774bf92fcaff55f7992"
+    }
+  ]
+}