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/aa6f9628c70d4b208e10a6734cd0fc02/_urls.json

@@ -0,0 +1,21 @@
+{
+  "detail_url": "https://oshwhub.com/eda_sbnvoqqej/kai-yuan-stm32-si-zhou-fei-xing-qi-xin-ban",
+  "cover_url": "https://image.lceda.cn/oshwhub/4215592dcecf453aa7e6b38b3481687b.jpg",
+  "attachments": [
+    {
+      "name": "匿名地面站V4.rar",
+      "url": "https://image.lceda.cn/attachments/2022/8/VnE1lJkBpzZUhpcGumewnHbWkYHBFItNmEZvhXwK.rar",
+      "original_id": "285ccc9b14b24b6e8aa8c37d5d6785ac"
+    },
+    {
+      "name": "电机座.stl",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/5998944dce07477289df0484850da4bc.stl",
+      "original_id": "b682876d20624fbbbca1d44c4c41df7d"
+    },
+    {
+      "name": "演示视频 .mp4",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/58c6fb9a64234b9194359dade3c34300.mp4",
+      "original_id": "15213b5f3eac40f883541fef3312c9ac"
+    }
+  ]
+}