fix(backend/db): Keep CreditTransaction entries on User delete (#10917)

This is a non-critical improvement for bookkeeping purposes.

- Change `CreditTransaction` <- `User` relation to `ON DELETE NO ACTION`
so that `CreditTransactions` are not automatically deleted when we
delete a user's data.

- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Migration applies without problems

autogpt_platform/backend/migrations/20250912164006_keep_credit_transactions_on_user_delete/migration.sql

@@ -0,0 +1,3 @@
+-- Re-create foreign key CreditTransaction <- User with ON DELETE NO ACTION
+ALTER TABLE "CreditTransaction" DROP CONSTRAINT "CreditTransaction_userId_fkey";
+ALTER TABLE "CreditTransaction" ADD CONSTRAINT "CreditTransaction_userId_fkey" FOREIGN KEY ("userId") REFERENCES "User"("id") ON DELETE NO ACTION ON UPDATE CASCADE;

autogpt_platform/backend/schema.prisma

@@ -528,7 +528,7 @@ model CreditTransaction {
   createdAt      DateTime @default(now())
 
   userId String
-  User   User   @relation(fields: [userId], references: [id], onDelete: Cascade)
+  User   User?   @relation(fields: [userId], references: [id], onDelete: NoAction)
 
   amount Int
   type   CreditTransactionType