Add js parameter for custom JavaScript injection

- Add js parameter to ScreenshotOptions interface (max 5000 chars)
- Execute JavaScript via page.evaluate() after delay, before CSS/hideSelectors
- 5-second timeout with JS_TIMEOUT error handling
- JS_EXECUTION_ERROR for script failures with sanitized error messages
- Support in both GET and POST endpoints with validation
- Updated OpenAPI spec for both GET and POST routes
- Added comprehensive test coverage (service + route layers)
- Updated SDK documentation (Node.js and Python) with examples

Test results: 414 tests passing (includes new JS injection tests)

sdk/node/README.md

@@ -121,6 +121,37 @@ const combined = await snap.capture({
 });
 ```
 
+### JavaScript Injection
+
+```typescript
+// Execute custom JavaScript before capture
+const interactiveScreenshot = await snap.capture({
+  url: 'https://example.com',
+  js: `
+    // Dismiss modal popup
+    document.querySelector('.modal-overlay')?.remove();
+    
+    // Scroll to specific content
+    window.scrollTo(0, 500);
+    
+    // Click button to reveal content
+    document.querySelector('#show-more-btn')?.click();
+    
+    // Wait for animation to complete
+    await new Promise(resolve => setTimeout(resolve, 1000));
+  `,
+});
+
+// Combine with other options for complex scenarios
+const complexCapture = await snap.capture({
+  url: 'https://example.com/app',
+  js: 'document.querySelector(".sidebar").style.display = "none";',
+  css: 'body { zoom: 0.8 }',
+  waitForSelector: '#content-loaded',
+  hideSelectors: ['.ad-banner', '.cookie-notice'],
+});
+```
+
 ## API Reference
 
 ### `new SnapAPI(apiKey, config?)`