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>

.gitignore

@@ -45,3 +45,6 @@ yarn-error.log*
 next-env.d.ts
 
 /src/generated/prisma
+
+# Claude Code local plugins (external git repos, не коммитим)
+.claude/plugins/