fix: address multiple bugs found in code review

- Fix dirty-user flush race: discard IDs individually after successful save
- Escape LIKE wildcards in LLM-generated topic keywords for DB queries
- Anonymize absent-member aliases to prevent LLM de-anonymization
- Pass correct MIME type to vision model based on image file extension
- Use enumerate instead of list.index() in bcs-scan loop
- Allow bot @mentions with non-report intent to fall through to moderation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cogs/chat.py

@@ -273,11 +273,14 @@ class ChatCog(commands.Cog):
                 image_attachment.filename,
                 user_text[:80],
             )
+            ext = image_attachment.filename.rsplit(".", 1)[-1].lower() if "." in image_attachment.filename else "png"
+            mime = f"image/{'jpeg' if ext == 'jpg' else ext}"
             response = await self.bot.llm_heavy.analyze_image(
                 image_bytes,
                 IMAGE_ROAST,
                 user_text=user_text,
                 on_first_token=start_typing,
+                media_type=mime,
             )
         else:
             # --- Text-only path: normal chat ---