fix(db): migration 0013 rename mascot_chat_messages to chatbot_messages

/api/chat/history 500 (relation "chatbot_messages" does not exist):
codebase di-rename mascot->chatbot (977a6f9) tapi tabel DB tidak
pernah di-migrate — backend SELECT dari chatbot_messages, DB masih
mascot_chat_messages dengan kolom mascot_response.

- Migration 0013 (idempotent): ALTER TABLE mascot_chat_messages RENAME
  TO chatbot_messages, RENAME COLUMN mascot_response -> bot_response,
  RENAME INDEX -> idx_chatbot_messages_user_created; journal when >
  max(created_at) di __drizzle_migrations (0012)
- Diterapkan live via psql; riwayat chat lama tetap ada
- Verifikasi: GET /api/chat/history 200 + data, POST /api/chat 200
  + tersimpan (total history 1 -> 2)

services/discord-gateway/drizzle/migrations/meta/_journal.json

@@ -92,6 +92,13 @@
       "when": 1785500000000,
       "tag": "0012_add_embedding_to_text_cache",
       "breakpoints": true
+    },
+    {
+      "idx": 13,
+      "version": "7",
+      "when": 1785551832190,
+      "tag": "0013_rename_mascot_chat_to_chatbot",
+      "breakpoints": true
     }
   ]
 }