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/126f1a61e36a4e859cd7a0c15ddcefe5/metadata.json

@@ -0,0 +1,86 @@
+{
+  "source": "oshwhub",
+  "source_url": "https://oshwhub.com/lxu0423/lithermal-thermal-imaging-camera",
+  "project_id": "126f1a61e36a4e859cd7a0c15ddcefe5",
+  "title": "[LiThermal] 热成像相机",
+  "description_short": "基于全志T113-S3和海康威视4117的热成像相机",
+  "description_path": "description.md",
+  "author": {
+    "username": "lxu0423",
+    "display_name": "小李电子实验室",
+    "user_id": "19aab6b29e4b41dc8c821d2a6c008f02"
+  },
+  "license": "GPL 3.0",
+  "tags": [],
+  "created_at": "2024-09-09T09:41:55.642Z",
+  "updated_at": "2025-03-23T22:11:50.000Z",
+  "published_at": "2024-12-30T01:07:54.000Z",
+  "crawled_at": "2026-04-28T17:00:38.569166+00:00",
+  "metrics": {
+    "likes": 634,
+    "stars": 1482,
+    "forks": 0,
+    "views": 136810,
+    "watch": 0,
+    "comments": 575
+  },
+  "cover": {
+    "url": "https://image.lceda.cn/oshwhub/80e9753174914f49b84889f0d552b457.jpg",
+    "path": "cover.jpg"
+  },
+  "files": [
+    {
+      "name": "速览.mp4",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/68a0c7c19526423a8d2a9bb936334bcb.mp4",
+      "original_id": "3d71e1f4b89947c3872c2144532a96bf",
+      "ext": "mp4",
+      "mime": "video/mp4",
+      "size": 30705240,
+      "md5": "4130e7ce9444c34e36ad5c07c30d2a9e"
+    },
+    {
+      "name": "相关资料.zip.003",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/c323c1ad023c42a6a416cf43570d6b6f.003",
+      "original_id": "6d5bb3d2313944549f1ee09d3b2f87ef",
+      "ext": "003",
+      "mime": "application/octet-stream",
+      "size": 6439964,
+      "md5": "e2b118bca3469aed75c9713d09b1102c"
+    },
+    {
+      "name": "相关资料.zip.001",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/0c1e6378acad41128eefc066f2c1cd0e.001",
+      "original_id": "76c65c28cd9b49dc8eb6c71c3187055c",
+      "ext": "001",
+      "mime": "application/octet-stream",
+      "size": 51380224,
+      "md5": "a8981a91fac46504d2b1af6e2f160027"
+    },
+    {
+      "name": "相关资料.zip.002",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/0aae986e4d344b14a2447e30d680b251.002",
+      "original_id": "4873e69741e2425797f7f9de38176a59",
+      "ext": "002",
+      "mime": "application/octet-stream",
+      "size": 51380224,
+      "md5": "f840d2fa5f0c502f87a0192fb5b48e2c"
+    },
+    {
+      "name": "UDISK.tar",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/c52ff45af58c4c75bfb791eba47e9f47.tar",
+      "original_id": "5ad9f5e1e3744734ba9b435bcb2b1e2a",
+      "ext": "tar",
+      "mime": "application/x-tar",
+      "size": 1658880,
+      "md5": "41cf031e179ba16c248d84a06b1288f6"
+    }
+  ],
+  "raw_fields": {
+    "path": "lxu0423/lithermal-thermal-imaging-camera",
+    "grade": 4,
+    "origin": "pro",
+    "public": true,
+    "publish": true,
+    "skipped_files": []
+  }
+}