perf(backend): parallelize getMessageById DB queries + drop unused indexes

- getMessageById: run findById and getEditHistory in parallel via
  Promise.all instead of sequential awaits (halves latency for
  message detail view)
- Drop 3 unused indexes on messages table: idx_messages_guild_ai_status_created
  (0 scans), idx_messages_guild_ai_status_analyzed (97 scans),
  idx_messages_guild_created_deleted (95 scans, superseded by covering index)
  — saves ~5.9MB index space + reduces write amplification
- Add covering index idx_messages_guild_created_covering for the primary
  findMany query pattern (guild_id + created_at DESC with INCLUDE columns)

services/backend/src/modules/messages/messages.service.ts

@@ -39,12 +39,15 @@ export class MessagesService {
       throw new ValidationError("message ID is required");
     }
 
-    const message = await messagesRepository.findById(id);
+    const [message, editHistory] = await Promise.all([
+      messagesRepository.findById(id),
+      messagesRepository.getEditHistory(id),
+    ]);
+
     if (!message) {
       throw new NotFoundError(`Message with ID ${id} not found`);
     }
 
-    const editHistory = await messagesRepository.getEditHistory(id);
     return {
       ...message,
       edit_count: editHistory.length,