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/e5dc20d57ef243b985ff86c12a46135c/_urls.json

@@ -0,0 +1,36 @@
+{
+  "detail_url": "https://oshwhub.com/kirito/rtl8367-4ge-2sfp_copy",
+  "cover_url": "https://image.lceda.cn/pullimage/UoqTBBjftYyim6eEWgYwvOhyJBkGwJi3jt8xOu65.jpeg",
+  "attachments": [
+    {
+      "name": "RTL8367S-4GE+SFP.bin",
+      "url": "https://image.lceda.cn/attachments/2023/3/FFOIRSzIBjcvynDS5pfMeBvHrAZy8cnSi2q9C5D9.bin",
+      "original_id": "05cf524ea2b64f9e8a963c4907d65f3b"
+    },
+    {
+      "name": "PCB_RTL8367-4GE+SFP_rev0.rar",
+      "url": "https://image.lceda.cn/attachments/2023/3/7gsjWk2s5mKrxqeZamXbajUYOhMs5wYRWRZREhRC.rar",
+      "original_id": "bcef653831474687868787bb82c54f85"
+    },
+    {
+      "name": "VID_20230312_133307.mp4",
+      "url": "https://image.lceda.cn/attachments/2023/3/xOj5ARdFTkrinmEc8imrKHQCyVf7a9klzulmi8E8.mp4",
+      "original_id": "890478d565c44260b1e590df6dd6ff53"
+    },
+    {
+      "name": "VID_20230312_135223.mp4",
+      "url": "https://image.lceda.cn/attachments/2023/3/q6WU25dXhfIkvt8DpH8euvpAtme4Bsy0LxoxZUrp.mp4",
+      "original_id": "997d1d2d32a34a9d8a9be619d30bfe02"
+    },
+    {
+      "name": "RTL8367S-4GE1SFP.dwg",
+      "url": "https://image.lceda.cn/attachments/2023/3/b7VRfNgfOQSRCPT4iAO79gr1toiFIhiwtmrq1Llb.dwg",
+      "original_id": "77da8e238d7b4b04860effb41dd38e1c"
+    },
+    {
+      "name": "装配体.STL",
+      "url": "https://image.lceda.cn/attachments/2023/3/E4mTk2Ej2FCguQpYIIEyYXhrNJrG5EEknodjdwHx.bin",
+      "original_id": "6edc4049be5744a898ecb80b3f4b7433"
+    }
+  ]
+}