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

README.md

@@ -138,6 +138,10 @@ ai_moderation:
   check_video_thumbnails: false      # Skip video thumbnails
   url_image_check_enabled: false     # Skip URL image downloads
 
+# User blocklist (blocks ALL media from specific users)
+blocked_user_ids:
+  - 123456789012345678  # Discord user ID to block
+
 # Known NSFW video domains (auto-block)
 nsfw_video_domains:
   - pornhub.com
@@ -160,6 +164,12 @@ nsfw_video_domains:
 - `duplicate_threshold`: How many duplicate messages trigger action
 - `mention_limit`: Max @mentions allowed per message
 
+**User Blocklist:**
+- `blocked_user_ids`: List of Discord user IDs to block
+- Automatically deletes ALL images, GIFs, embeds, and URLs from these users
+- No AI cost - instant deletion
+- Useful for known problematic users or spam accounts
+
 **Cost Controls:**
 The bot includes multiple layers of cost control:
 - Rate limiting (25 AI checks/hour/guild, 5/hour/user by default)
@@ -222,6 +232,12 @@ guardden/
 
 ## How It Works
 
+### User Blocklist (Instant, No AI Cost)
+1. Checks if message author is in `blocked_user_ids` list
+2. If message contains ANY media (images, embeds, URLs), instantly deletes it
+3. No AI analysis needed - immediate action
+4. Useful for known spam accounts or problematic users
+
 ### Spam Detection
 1. Bot monitors message rate per user
 2. Detects duplicate messages
@@ -229,12 +245,13 @@ guardden/
 4. Violations result in message deletion + timeout
 
 ### NSFW Image Detection
-1. Bot checks attachments and embeds for images
-2. Applies rate limiting and deduplication
-3. Downloads image and sends to AI provider
-4. AI analyzes for NSFW content categories
-5. Violations result in message deletion + timeout
-6. Optionally checks known NSFW video domain links
+1. Checks user blocklist first (instant deletion if matched)
+2. Checks NSFW video domain blocklist (instant deletion)
+3. Bot checks attachments and embeds for images
+4. Applies rate limiting and deduplication
+5. Downloads image and sends to AI provider
+6. AI analyzes for NSFW content categories
+7. Violations result in message deletion + timeout
 
 ### Cost Management
 The bot includes aggressive cost controls for AI usage: