feat: add POST /v1/screenshots/batch endpoint

- Batch screenshot endpoint: take 1-10 screenshots in a single request
- Concurrent processing with Promise.allSettled (partial success support)
- Upfront quota check for all URLs before processing
- Per-URL SSRF validation via existing takeScreenshot()
- Added incrementUsage() to usage middleware for granular tracking
- 10 new tests covering all edge cases
- Updated OpenAPI docs (JSDoc on route)
- Updated Node.js and Python SDK READMEs with batch method docs

sdk/python/README.md

@@ -225,6 +225,29 @@ except SnapAPIError as e:
 | `css` | `str` | — | Custom CSS to inject before capture (max 5000 chars) |
 | `clip` | `dict` | — | Crop rectangle: `{"x": int, "y": int, "width": int, "height": int}` (mutually exclusive with full_page/selector) |
 
+### `snap.batch(urls, **options) -> list[dict]`
+
+Take multiple screenshots in a single request. Each URL counts as one screenshot toward usage limits.
+
+```python
+results = snap.batch(
+    ["https://example.com", "https://example.org"],
+    format="jpeg", width=1920, height=1080
+)
+
+for result in results:
+    if result["status"] == "success":
+        with open(f"{result['url']}.jpg", "wb") as f:
+            f.write(base64.b64decode(result["image"]))
+    else:
+        print(f"Failed: {result['url']} — {result['error']}")
+```
+
+- **Max 10 URLs per batch**
+- All options (format, width, height, etc.) are shared across all URLs
+- Returns partial results — some may succeed while others fail
+- Response is always JSON with `{ "results": [...] }`
+
 ### `snap.health() -> dict`
 
 Returns API health status.