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/migrations/20260507120000_add_balance_transactions/migration.sql

@@ -0,0 +1,11 @@
+CREATE TABLE "BalanceTransaction" (
+  "id" TEXT NOT NULL,
+  "userId" TEXT NOT NULL,
+  "amount" DECIMAL(10,2) NOT NULL,
+  "description" TEXT NOT NULL,
+  "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  CONSTRAINT "BalanceTransaction_pkey" PRIMARY KEY ("id"),
+  CONSTRAINT "BalanceTransaction_userId_fkey" FOREIGN KEY ("userId") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE
+);
+
+CREATE INDEX "BalanceTransaction_userId_idx" ON "BalanceTransaction"("userId");