feat(database): add unique constraint on workspace subdomain (#9084)

Added a unique constraint to the "subdomain" column in the workspace
entity to ensure no duplicate subdomains exist in the database. Included
a TypeORM migration script to enforce this change at the database level.

packages/twenty-server/src/engine/core-modules/workspace/workspace.entity.ts

@@ -144,7 +144,7 @@ export class Workspace {
   databaseSchema: string;
 
   @Field()
-  @Column()
+  @Column({ unique: true })
   subdomain: string;
 
   @Field()