Add CSS background image extraction feature (Issue #1691)

[YxmMyth]

This commit adds support for extracting CSS background images during crawling, addressing issue #1691 where background images were being skipped.

Changes

New Files

• crawl4ai/js_snippet/extract_css_backgrounds.js: JavaScript script to extract background images from computed styles in the browser

Modified Files

• crawl4ai/models.py:

  • Added css_images field to Media class
  • Added css_images_data field to AsyncCrawlResponse

• crawl4ai/async_configs.py:

  • Added CSS background image configuration parameters to CrawlerRunConfig:
    • extract_css_images (bool, default False)
    • css_image_min_width (int, default 100)
    • css_image_min_height (int, default 100)
    • css_image_score_threshold (int, default 2)
    • css_exclude_repeating (bool, default True)

• crawl4ai/content_scraping_strategy.py:

  • Added process_css_background_images() method
  • Integrated CSS image extraction into _process_element()
  • Added css_images to media dictionary

• crawl4ai/async_crawler_strategy.py:

  • Added JavaScript execution in _crawl_web() to extract CSS backgrounds
  • Included css_images_data in AsyncCrawlResponse

• crawl4ai/async_webcrawler.py:

  • Modified aprocess_html() to accept and pass css_images_data
  • Added Dict type import

Features

• Extracts background images from both inline styles and stylesheets
• Uses window.getComputedStyle() for accurate extraction
• Smart filtering (small elements, repeating patterns)
• Scoring system based on element size and properties
• Opt-in by default for backward compatibility
• Separate storage in media.css_images

Usage

result = await crawler.arun(
    url="https://example.com",
    extract_css_images=True,
    css_image_min_width=100,
    css_image_min_height=100,
)

css_images = result.media.get('css_images', [])

Closes #1691