refactor: migrate authentication from Supabase to Better Auth and update project configuration

apps/backend/prisma/user.prisma

@@ -1,11 +1 @@
-// model User {
-//   id             String        @id @db.Uuid
-//   supabaseUserId String        @unique @map("supabase_user_id") @db.Uuid
-//   email          String        @unique
-//   fullName       String        @map("full_name")
-//   createdAt      DateTime      @default(now()) @map("created_at")
-//   updatedAt      DateTime      @updatedAt @map("updated_at")
-//   complexes      ComplexUser[]
-
-//   @@map("users")
-// }
+// User model is defined in auth.prisma (Better Auth schema).