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/python/README.md

@@ -125,6 +125,37 @@ combined = snap.capture(
 )
 ```
 
+### JavaScript Injection
+
+```python
+# Execute custom JavaScript before capture
+interactive_screenshot = snap.capture(
+    "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
+complex_capture = snap.capture(
+    "https://example.com/app",
+    js='document.querySelector(".sidebar").style.display = "none";',
+    css="body { zoom: 0.8 }",
+    wait_for_selector="#content-loaded",
+    hide_selectors=[".ad-banner", ".cookie-notice"],
+)
+```
+
 ### Error Handling
 
 ```python