rebuild.py: How AIVN Extracts Unique Articles and Rebuilds Feeds

AIVN's rebuild.py performs three functions: extracts unique articles from all batch files (deduplication), rebuilds feed.xml with the 1,000 most recent articles (RSS), and regenerates sitemap.xml with all article URLs (SEO). The unique count from rebuild output is the authoritative article count — more reliable than summing batch inserts.