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/5e03b6545745463bb8b64813209ffb8c/metadata.json

@@ -0,0 +1,77 @@
+{
+  "source": "oshwhub",
+  "source_url": "https://oshwhub.com/course-examples/yi-qi-yi-biao-jian-yi-shu-zi-shi-bo-qi-she-ji-cha-jian-ban",
+  "project_id": "5e03b6545745463bb8b64813209ffb8c",
+  "title": "简易数字示波器设计（入门版）",
+  "description_short": "打造一套针对电子爱好者及学生学习数字示波器的基础项目，以单片机核心板与插件元器件结合起来，十分适合入门学习！",
+  "description_path": "description.md",
+  "author": {
+    "username": "course-examples",
+    "display_name": "EDA课程案例团队",
+    "user_id": "ebbda44292b4423b92db6e999d50f4d4"
+  },
+  "license": "GPL 3.0",
+  "tags": [],
+  "created_at": "2023-12-25T08:15:57.000Z",
+  "updated_at": "2026-04-05T03:44:46.000Z",
+  "published_at": "2026-01-16T09:07:45.000Z",
+  "crawled_at": "2026-04-28T17:00:16.524061+00:00",
+  "metrics": {
+    "likes": 708,
+    "stars": 1824,
+    "forks": 0,
+    "views": 199957,
+    "watch": 0,
+    "comments": 390
+  },
+  "cover": {
+    "url": "https://image.lceda.cn/pullimage/vZ4gYlm4aPajQfTGsAfyIPsZu12NXrt3CZIDGgGx.jpeg",
+    "path": "cover.jpeg"
+  },
+  "files": [
+    {
+      "name": "简易数字示波器焊接文档.pdf",
+      "url": "https://image.lceda.cn/attachments/2024/1/kxWXt8GSAQ8m7qzU4GvzzPgvxt3mFEBf8VAnQUxn.pdf",
+      "original_id": "3ce9b9717a674de1a37984091d7af240",
+      "ext": "pdf",
+      "mime": "application/pdf",
+      "size": 731912,
+      "md5": "f862eee2ead48f505435c87419e49811"
+    },
+    {
+      "name": "物料清单-简易数字示波器.xlsx",
+      "url": "https://image.lceda.cn/attachments/2024/1/HWVhBxnWjIL06wyudbUSIlaQjCMn1HyjBhldkK2Q.bin",
+      "original_id": "5f5bee4bbeb64bd088c507c5666c0d9e",
+      "ext": "bin",
+      "mime": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
+      "size": 11632,
+      "md5": "94b7687aba7469bcc3cc1a4f6b552c57"
+    },
+    {
+      "name": "PCB焊接辅助工具-简易数字示波器V1.2.html",
+      "url": "https://image.lceda.cn/attachments/2024/1/9NgRZm8f6DeFWEJuZrEAqwWCDmlEEJMf9GHYpRJz.html",
+      "original_id": "02c7b147329b458aa3d0079ccef6cae7",
+      "ext": "html",
+      "mime": "text/html",
+      "size": 7115269,
+      "md5": "35f45c7e852270520c52174e2a27afc3"
+    },
+    {
+      "name": "简易数字示波器-装配图.pdf",
+      "url": "https://image.lceda.cn/oshwhub/project/attachments/4ef4a80fb8bc40ce9fe6de7bde763661.pdf",
+      "original_id": "5b5b0535cdb74cbc9342e99f2b73aa14",
+      "ext": "pdf",
+      "mime": "application/pdf",
+      "size": 2599352,
+      "md5": "9119ea99e4ec80cddf19dfb28c9df11d"
+    }
+  ],
+  "raw_fields": {
+    "path": "course-examples/yi-qi-yi-biao-jian-yi-shu-zi-shi-bo-qi-she-ji-cha-jian-ban",
+    "grade": 4,
+    "origin": "pro",
+    "public": true,
+    "publish": true,
+    "skipped_files": []
+  }
+}