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/d53cd4f3355c45319a36b19cc9ff252e/_urls.json

@@ -0,0 +1,16 @@
+{
+  "detail_url": "https://oshwhub.com/zl4430/mpu-6500-280",
+  "cover_url": "https://image.lceda.cn/pullimage/kVn5PSaVguqoyJHRLrVBiqmm9y9mohfjeUNv97so.jpeg",
+  "attachments": [
+    {
+      "name": "左手边 F405 MPU 6500 试飞成功 部分功能待测试版 (2).zip",
+      "url": "https://image.lceda.cn/attachments/2021/12/OSfnDfGqKVQlgJLVankEWRB2ja0WbZFqrNvHxWGQ.zip",
+      "original_id": "f312429674e245208262d1e1b7b5c87c"
+    },
+    {
+      "name": "F405+6500陀螺仪飞控 20 30双版本含固件和配置文件左手边.zip",
+      "url": "https://image.lceda.cn/attachments/2022/4/IVajraS7oaHGtqfjVGjkgRXsVtTU5KJJFW9BRYiN.zip",
+      "original_id": "20601577108b423b8cb3c4971d93ee81"
+    }
+  ]
+}