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/ce949d32c86f448398bd5fdbeb026a77/metadata.json

@@ -0,0 +1,50 @@
+{
+  "source": "oshwhub",
+  "source_url": "https://oshwhub.com/goodyuhuang/ji-yu-VL813de-USB3.0-HUBshe-ji",
+  "project_id": "ce949d32c86f448398bd5fdbeb026a77",
+  "title": "【电路已验证】基于VL813的USB3.0-HUB设计",
+  "description_short": "一个使用TYPE-C接口作为数据输入的USB3.0-HUB（立创EDA为直接导入，未作DRC检查，请不要直接打样，可使用附件资料打样）",
+  "description_path": "description.md",
+  "author": {
+    "username": "goodyuhuang",
+    "display_name": "GoodYuHuang",
+    "user_id": "53adcecec2b840069aab2ceb81e52d0e"
+  },
+  "license": "GPL 3.0",
+  "tags": [],
+  "created_at": "2022-11-20T09:13:05.000Z",
+  "updated_at": "2026-02-05T02:04:02.000Z",
+  "published_at": "2026-01-27T06:18:21.000Z",
+  "crawled_at": "2026-04-28T17:27:58.907851+00:00",
+  "metrics": {
+    "likes": 462,
+    "stars": 1113,
+    "forks": 290,
+    "views": 91780,
+    "watch": 0,
+    "comments": 233
+  },
+  "cover": {
+    "url": "https://image.lceda.cn/pullimage/dvsAxKbsoMGFjlggZHurDkTZy6M0ftamuxpKXW0H.jpeg",
+    "path": null
+  },
+  "files": [
+    {
+      "name": "USB3.0-HUB设计V1.2.zip",
+      "url": "https://image.lceda.cn/attachments/2023/9/ess2KhkUToEo0os7mlsPlCX7Qj21Wk7vnuGh8z13.zip",
+      "original_id": "10b3e9c4e33445b1a96983e2ae4e341a",
+      "ext": "zip",
+      "mime": "application/x-zip-compressed",
+      "size": 35170629,
+      "md5": "f6dab9866fe8138c83562fa0c7b1b307"
+    }
+  ],
+  "raw_fields": {
+    "path": "goodyuhuang/ji-yu-VL813de-USB3.0-HUBshe-ji",
+    "grade": 4,
+    "origin": "std",
+    "public": true,
+    "publish": true,
+    "skipped_files": []
+  }
+}