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/b38847543af34f64bd034e24c94c130e/metadata.json

@@ -0,0 +1,68 @@
+{
+  "source": "oshwhub",
+  "source_url": "https://oshwhub.com/aeromodeller/superuav",
+  "project_id": "b38847543af34f64bd034e24c94c130e",
+  "title": "SuperUAV",
+  "description_short": "SUAV开源四轴无人机。不止是开源，从点灯开始，一步步手搓飞控，最终完美飞行。授人以鱼，更授人予渔。",
+  "description_path": "description.md",
+  "author": {
+    "username": "aeromodeller",
+    "display_name": "Aeromodeller",
+    "user_id": "e8a5c199c7c54f5989d704a8f8e7eaa6"
+  },
+  "license": "GPL 3.0",
+  "tags": [],
+  "created_at": "2022-11-11T02:32:04.000Z",
+  "updated_at": "2026-03-11T09:15:59.000Z",
+  "published_at": "2022-12-05T01:04:40.000Z",
+  "crawled_at": "2026-04-28T17:30:23.058651+00:00",
+  "metrics": {
+    "likes": 481,
+    "stars": 1117,
+    "forks": 198,
+    "views": 57303,
+    "watch": 0,
+    "comments": 78
+  },
+  "cover": {
+    "url": "https://image.lceda.cn/pullimage/DdYgeOd2X0egqLY0Jc4jCHe6Spyj4PfNrPEqmmHk.jpeg",
+    "path": null
+  },
+  "files": [
+    {
+      "name": "suav_v2.0_源码资料包.rar",
+      "url": "https://image.lceda.cn/attachments/2022/11/4F4JeiQBlm9z3vLroA2Wmp4LjCSeqMxsAJVVog5d.rar",
+      "original_id": "ebd30978bd6441bea44bf100f9c8cbde",
+      "ext": "rar",
+      "mime": "application/octet-stream",
+      "size": 46918854,
+      "md5": "fca4e46b8f1b294158d18037c9fa9bbe"
+    },
+    {
+      "name": "12_开源啦_suav.mp4",
+      "url": "https://image.lceda.cn/attachments/2022/11/fIFbElLxCXiGeiZGjUDUaTsBomXIPUCaBngkkSXV.mp4",
+      "original_id": "1fb9618a8749420ba8c9bd9c0757c70b",
+      "ext": "mp4",
+      "mime": "video/mp4",
+      "size": 29993211,
+      "md5": "3eeaf742bc4a4449b468630b12bf3d6e"
+    },
+    {
+      "name": "suav dinggao xiao.mp4",
+      "url": "https://image.lceda.cn/attachments/2022/12/7bYJxQrEuuVEVLDdQedceugAHIL3LheE6gpn0Jvk.qt",
+      "original_id": "60757e60b2a8466f90fe98fa3c07da5e",
+      "ext": "qt",
+      "mime": "video/mp4",
+      "size": 49076013,
+      "md5": "915672b6998b45221be36447a122cfa7"
+    }
+  ],
+  "raw_fields": {
+    "path": "aeromodeller/superuav",
+    "grade": 4,
+    "origin": "std",
+    "public": true,
+    "publish": true,
+    "skipped_files": []
+  }
+}