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/node/README.md

@@ -216,6 +216,30 @@ Returns a `Promise<Buffer>` containing the screenshot image.
 | `css` | `string` | — | Custom CSS to inject before capture (max 5000 chars) |
 | `clip` | `object` | — | Crop rectangle: `{x, y, width, height}` (mutually exclusive with fullPage/selector) |
 
+### `snap.batch(urls, options?)`
+
+Take multiple screenshots in a single request. Each URL counts as one screenshot toward usage limits.
+
+```typescript
+const results = await snap.batch(
+  ['https://example.com', 'https://example.org'],
+  { format: 'jpeg', width: 1920, height: 1080 }
+);
+
+for (const result of results) {
+  if (result.status === 'success') {
+    fs.writeFileSync(`${result.url}.jpg`, Buffer.from(result.image, 'base64'));
+  } else {
+    console.error(`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()`
 
 Returns API health status.