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/c8142997cb5c4e43acc5be42f54be6d2/metadata.json

@@ -0,0 +1,50 @@
+{
+  "source": "oshwhub",
+  "source_url": "https://oshwhub.com/expert/gai-jin-xin-exlink-duo-gong-neng-diao-shi-qi-fen-li-die-ban",
+  "project_id": "c8142997cb5c4e43acc5be42f54be6d2",
+  "title": "Exlink最好用的嵌入式多功能调试器",
+  "description_short": "Exlink是一款基于ESP32S3设计的优雅的嵌入式多功能调试器，具有DAPlink、逻辑分析仪、串口助手、数控电源、无线下载器、无线串口、简易示波器等十余种不同功能，涵盖大部分嵌入式调试场景。",
+  "description_path": "description.md",
+  "author": {
+    "username": "expert",
+    "display_name": "Expert电子实验室",
+    "user_id": "33e19026ed794dc99d07c4686fe7eb17"
+  },
+  "license": "CC BY-NC-SA 3.0",
+  "tags": [],
+  "created_at": "2024-09-23T16:22:19.000Z",
+  "updated_at": "2026-03-18T07:05:51.000Z",
+  "published_at": "2025-08-25T02:11:58.000Z",
+  "crawled_at": "2026-04-28T17:00:05.376725+00:00",
+  "metrics": {
+    "likes": 804,
+    "stars": 2024,
+    "forks": 0,
+    "views": 135854,
+    "watch": 0,
+    "comments": 530
+  },
+  "cover": {
+    "url": "https://image.lceda.cn/oshwhub/ee5d2d79720249efa47d42b06483ead2.png",
+    "path": "cover.png"
+  },
+  "files": [
+    {
+      "name": "VID20241019154253.mp4",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/be2af8ade36f4503ad6f0c8570709ed1.mp4",
+      "original_id": "ba894e8c86fa4b17b41abba640c199ac",
+      "ext": "mp4",
+      "mime": "video/mp4",
+      "size": 30984464,
+      "md5": "e4a1a9d6c49df43cb84159175f21dd46"
+    }
+  ],
+  "raw_fields": {
+    "path": "expert/gai-jin-xin-exlink-duo-gong-neng-diao-shi-qi-fen-li-die-ban",
+    "grade": 4,
+    "origin": "pro",
+    "public": true,
+    "publish": true,
+    "skipped_files": []
+  }
+}