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/0bc142e0f24e4b06ab6d555f935eca98/metadata.json

@@ -0,0 +1,68 @@
+{
+  "source": "oshwhub",
+  "source_url": "https://oshwhub.com/clz1/5.8Ghztu-zhuan",
+  "project_id": "0bc142e0f24e4b06ab6d555f935eca98",
+  "title": "飞行器设计之5.8Ghz图传",
+  "description_short": "此工程主要是进行飞行器的5.8Ghz图传设计，实现飞行器上摄像头视频信号的发射。主要包括原理图设计、PCB设计和电路仿真。原理图和PCB设计最多占此工程工作量的30%，电路仿真是非常耗时且繁琐过程。",
+  "description_path": "description.md",
+  "author": {
+    "username": "clz1",
+    "display_name": "DroneCYF",
+    "user_id": "cf041e4192a545a4a2767f7cde799341"
+  },
+  "license": "CC-BY-NC-SA 3.0",
+  "tags": [],
+  "created_at": "2022-09-13T02:38:49.000Z",
+  "updated_at": "2026-01-31T08:24:49.000Z",
+  "published_at": "2023-09-11T07:13:46.000Z",
+  "crawled_at": "2026-04-28T17:24:02.041754+00:00",
+  "metrics": {
+    "likes": 776,
+    "stars": 1271,
+    "forks": 0,
+    "views": 81317,
+    "watch": 0,
+    "comments": 184
+  },
+  "cover": {
+    "url": "https://image.lceda.cn/pullimage/rKZs6yrLjl0v060PRD6pKJvTSms8ezjeh4lIlcq4.jpeg",
+    "path": "cover.jpeg"
+  },
+  "files": [
+    {
+      "name": "BOM.xls",
+      "url": "https://image.lceda.cn/attachments/2022/10/kxGEfi0eDAUog7uxyoPzgGfXCJeyGhdp0aCZN4Nz",
+      "original_id": "82d8af7950774f60a12696da6d7e5228",
+      "ext": null,
+      "mime": "application/vnd.ms-excel",
+      "size": 27648,
+      "md5": "c89e770cc91dcffe70b99a481876b80e"
+    },
+    {
+      "name": "射频电路设计  理论与应用.pdf",
+      "url": "https://image.lceda.cn/attachments/2022/10/BCeWSzeoEicDMmYJC4L6C6cpmgJe7Klx1x2Hrope.pdf",
+      "original_id": "f7b910d03acd455c9d1823a391b7c69f",
+      "ext": "pdf",
+      "mime": "application/pdf",
+      "size": 2144267,
+      "md5": "67af74fc9448d1b51b0dbe9ace84f55f"
+    },
+    {
+      "name": "工程描述.pdf",
+      "url": "https://image.lceda.cn/attachments/2022/10/OCaxkHG8J74GJz1cymG2H1gPS3hakVGrxE6F0cTt.pdf",
+      "original_id": "4b126027f64a4510be450cd357a337a3",
+      "ext": "pdf",
+      "mime": "application/pdf",
+      "size": 9365415,
+      "md5": "ea3b5db86bbc0795b39044fd5c0c3172"
+    }
+  ],
+  "raw_fields": {
+    "path": "clz1/5.8Ghztu-zhuan",
+    "grade": 4,
+    "origin": "pro",
+    "public": true,
+    "publish": true,
+    "skipped_files": []
+  }
+}