fix(schema): add composite index for LlmModelMigration active queries

Adds @@index([sourceModelSlug, isReverted]) to match the migration SQL.
This prevents Prisma migration conflicts and optimizes queries for
non-reverted migrations by source model slug.

autogpt_platform/backend/schema.prisma

@@ -1289,6 +1289,7 @@ model LlmModelMigration {
   @@index([sourceModelSlug])
   @@index([targetModelSlug])
   @@index([isReverted])
+  @@index([sourceModelSlug, isReverted]) // Composite index for active migration queries
 }
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