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/c19a4fc2119f4a44860ccb55ed171e8e/metadata.json

@@ -0,0 +1,59 @@
+{
+  "source": "oshwhub",
+  "source_url": "https://oshwhub.com/youi/P150C-Pro-D-shu-kong-dian-zi-fu-zai",
+  "project_id": "c19a4fc2119f4a44860ccb55ed171e8e",
+  "title": "P150C Pro-D数控电子负载 电池容量测试仪",
+  "description_short": "P150C Pro-D系列直流恒流型数控电子负载是基于STC8H系列单片机为主控芯片开发而成，该产品支持±150V电压测量/15A电流测量/150W功率耗散，测量精度高达0.5%，并且支持多种测量模式",
+  "description_path": "description.md",
+  "author": {
+    "username": "youi",
+    "display_name": "cnwans",
+    "user_id": "3487f85b25e8466a98c86f32e6f9315d"
+  },
+  "license": "GPL 3.0",
+  "tags": [],
+  "created_at": "2023-03-20T13:28:17.000Z",
+  "updated_at": "2026-04-01T16:37:22.000Z",
+  "published_at": "2025-06-26T01:41:28.000Z",
+  "crawled_at": "2026-04-28T17:30:54.506725+00:00",
+  "metrics": {
+    "likes": 246,
+    "stars": 644,
+    "forks": 476,
+    "views": 78354,
+    "watch": 0,
+    "comments": 268
+  },
+  "cover": {
+    "url": "https://image.lceda.cn/pullimage/DiPddiHrzbLYEMB3Twl5qM0szhEeGOvojkc1X79a.jpeg",
+    "path": null
+  },
+  "files": [
+    {
+      "name": "操作演示视频.mp4",
+      "url": "https://image.lceda.cn/attachments/2023/3/Tz9DaCWWXcqfm8smLrYCTdFXBPsUO3znIdrQ0R38.mp4",
+      "original_id": "3788f7ca68de45a8829b33700df32bc1",
+      "ext": "mp4",
+      "mime": "video/mp4",
+      "size": 39322777,
+      "md5": "870f5f22a39d39ba1548fa153c9bbc8e"
+    },
+    {
+      "name": "挑战940W功率.mp4",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/05da80981c4742088892edae49d4ec86.mp4",
+      "original_id": "f44810fb07cb4974b2cdf13dd0db4a1a",
+      "ext": "mp4",
+      "mime": "video/mp4",
+      "size": 54888698,
+      "md5": "71924f16d633448ebd92846e81d9b90e"
+    }
+  ],
+  "raw_fields": {
+    "path": "youi/P150C-Pro-D-shu-kong-dian-zi-fu-zai",
+    "grade": 3,
+    "origin": "std",
+    "public": true,
+    "publish": true,
+    "skipped_files": []
+  }
+}