Add balance transactions to user admin panel

Introduces BalanceTransaction model to track per-user balance history
(prepayments, refunds, partner credits). Admin can add/delete transactions;
current balance is computed as the running sum.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -27,15 +27,16 @@ model User {
   createdAt     DateTime  @default(now())
   updatedAt     DateTime  @updatedAt
 
-  sessions        Session[]
-  accounts        Account[]
-  enrollments     CourseEnrollment[]
-  progress        LessonProgress[]
-  submissions     HomeworkSubmission[]
-  comments        LessonComment[]
-  feedbacks       HomeworkFeedback[]
-  accessLogs      AccessLog[] @relation("AccessLogUser")
-  adminAccessLogs AccessLog[] @relation("AccessLogAdmin")
+  sessions             Session[]
+  accounts             Account[]
+  enrollments          CourseEnrollment[]
+  progress             LessonProgress[]
+  submissions          HomeworkSubmission[]
+  comments             LessonComment[]
+  feedbacks            HomeworkFeedback[]
+  accessLogs           AccessLog[] @relation("AccessLogUser")
+  adminAccessLogs      AccessLog[] @relation("AccessLogAdmin")
+  balanceTransactions  BalanceTransaction[]
 }
 
 model Session {
@@ -311,6 +312,20 @@ model LessonComment {
   replies LessonComment[] @relation("CommentReplies")
 }
 
+// ─────────────────────────────────────────────
+// Balance
+// ─────────────────────────────────────────────
+
+model BalanceTransaction {
+  id          String   @id @default(cuid())
+  userId      String
+  amount      Decimal  @db.Decimal(10, 2)
+  description String
+  createdAt   DateTime @default(now())
+
+  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+}
+
 // ─────────────────────────────────────────────
 // Platform Settings (key-value store)
 // ─────────────────────────────────────────────