feat: Add user blocklist for instant media deletion

Add blocklist feature to automatically delete ALL media (images, GIFs,
embeds, URLs) from specific users without AI analysis.

Changes:
- Add blocked_user_ids config field to config.yml
- Implement blocklist check in ai_moderation.py (runs before AI checks)
- Update README.md with blocklist documentation

Benefits:
- No AI cost (instant deletion)
- Useful for known spam accounts or problematic users
- Blocks all media types: attachments, embeds, URLs
- Logged for moderation tracking

config.yml

@@ -36,6 +36,12 @@ ai_moderation:
   check_video_thumbnails: false       # Skip video thumbnails (disabled per user request)
   url_image_check_enabled: false      # Skip URL image downloads (disabled per user request)
 
+# User Blocklist (No AI cost)
+# Block all images, GIFs, embeds, and URLs from these users
+# Add Discord user IDs here
+blocked_user_ids:
+  # Example: - 123456789012345678
+
 # NSFW Video Domain Blocklist (No AI cost)
 # These domains are blocked instantly without AI analysis
 nsfw_video_domains: