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/f052d1b8ca4c48778b872b542c20d1c4/metadata.json

@@ -0,0 +1,59 @@
+{
+  "source": "oshwhub",
+  "source_url": "https://oshwhub.com/1378dm/better-pcb-picture",
+  "project_id": "f052d1b8ca4c48778b872b542c20d1c4",
+  "title": "更好的PCB照片 - 避免卡顿",
+  "description_short": "将任意图片转化成LCEDA的LIB文件，大大减少卡顿",
+  "description_path": "description.md",
+  "author": {
+    "username": "1378dm",
+    "display_name": "1378dm",
+    "user_id": "b2064b0a993b4f93aadc2893e1b656aa"
+  },
+  "license": "CC-BY-NC-SA 3.0",
+  "tags": [],
+  "created_at": "2022-01-16T06:52:32.000Z",
+  "updated_at": "2025-08-15T03:27:32.000Z",
+  "published_at": "2022-08-29T02:15:17.000Z",
+  "crawled_at": "2026-04-28T17:28:04.470620+00:00",
+  "metrics": {
+    "likes": 743,
+    "stars": 1330,
+    "forks": 49,
+    "views": 54307,
+    "watch": 0,
+    "comments": 142
+  },
+  "cover": {
+    "url": "https://image.lceda.cn/pullimage/O80U1HV9kRkbhYSQle5Bo8Ze76GnpWrpi6PXRYHY.jpeg",
+    "path": null
+  },
+  "files": [
+    {
+      "name": "pic2LCEDA_v0.2.z01",
+      "url": "https://image.lceda.cn/attachments/2022/2/5kMowvU6Kf2srNnR4RwAt20WYEzBzqc0tbwqmQC8.zip",
+      "original_id": "baa625ddfce84e2293f24d6310269282",
+      "ext": "zip",
+      "mime": "application/octet-stream",
+      "size": 50331648,
+      "md5": "f5291c3a35bd159389980ec3dcc07bf5"
+    },
+    {
+      "name": "pic2LCEDA_v0.2.zip",
+      "url": "https://image.lceda.cn/attachments/2022/2/hWSDWqZ0lMQ32gv6xiGswBJv8edpolGcFJN1PgkL.bin",
+      "original_id": "542a49b8307143a082c471e4a833ef57",
+      "ext": "bin",
+      "mime": "application/x-zip-compressed",
+      "size": 34416656,
+      "md5": "369eba5d51b8926d54fadb5bf0652d4b"
+    }
+  ],
+  "raw_fields": {
+    "path": "1378dm/better-pcb-picture",
+    "grade": 4,
+    "origin": "std",
+    "public": true,
+    "publish": true,
+    "skipped_files": []
+  }
+}