fix: add @@schema("platform") to LlmCostUnit enum

Sentry caught this - enums also need @@schema directive with multiSchema enabled.
Without it, Prisma looks for enum in public schema but it's created in platform.

autogpt_platform/backend/schema.prisma

@@ -1343,6 +1343,8 @@ model OAuthRefreshToken {
 enum LlmCostUnit {
   RUN
   TOKENS
+
+  @@schema("platform")
 }
 
 model LlmProvider {