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

src/guardden/cogs/ai_moderation.py

@@ -92,7 +92,28 @@ class AIModeration(commands.Cog):
         if not config.get_setting("ai_moderation.enabled", True):
             return
 
-        # Check NSFW video domain blocklist first (no AI cost)
+        # Check user blocklist first (blocks ALL media from specific users)
+        blocked_users = config.get_setting("blocked_user_ids", [])
+        if message.author.id in blocked_users:
+            # Check if message has any media content (images, embeds, URLs)
+            has_media = (
+                bool(message.attachments) or 
+                bool(message.embeds) or 
+                bool(URL_PATTERN.search(message.content))
+            )
+            
+            if has_media:
+                try:
+                    await message.delete()
+                    logger.info(
+                        f"Deleted media content from blocked user {message.author} "
+                        f"({message.author.id}) in {message.guild.name}"
+                    )
+                except (discord.Forbidden, discord.NotFound):
+                    logger.warning(f"Failed to delete message from blocked user {message.author.id}")
+                return
+
+        # Check NSFW video domain blocklist (no AI cost)
         if self._has_nsfw_video_link(message.content):
             try:
                 await message.delete()