fix(funds): 加固转账冲正、结算收付幂等与坏账核销

- 转入 main_site_timeout 等不可冲正场景直接拒绝，避免假结案
- payment_records / settlement_adjustments 增加 partial unique 索引
- 坏账核销行锁 + meta 幂等回放；补差单记录 result_bill_id
- 新增 FundOperationsHardeningTest 覆盖关键路径

database/migrations/2026_06_26_150000_harden_settlement_and_payment_idempotency.php

@@ -0,0 +1,55 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('settlement_adjustments', function (Blueprint $table): void {
+            $table->foreignId('result_bill_id')
+                ->nullable()
+                ->after('original_bill_id')
+                ->constrained('settlement_bills')
+                ->nullOnDelete();
+        });
+
+        Schema::table('payment_records', function (Blueprint $table): void {
+            $table->dropIndex(['settlement_bill_id', 'idempotency_key']);
+        });
+
+        Schema::table('settlement_adjustments', function (Blueprint $table): void {
+            $table->dropIndex(['original_bill_id', 'idempotency_key']);
+        });
+
+        DB::statement(
+            'CREATE UNIQUE INDEX payment_records_bill_idempotency_unique '
+            .'ON payment_records (settlement_bill_id, idempotency_key) '
+            ."WHERE idempotency_key IS NOT NULL AND idempotency_key <> ''",
+        );
+
+        DB::statement(
+            'CREATE UNIQUE INDEX settlement_adjustments_bill_idempotency_unique '
+            .'ON settlement_adjustments (original_bill_id, idempotency_key) '
+            ."WHERE idempotency_key IS NOT NULL AND idempotency_key <> ''",
+        );
+    }
+
+    public function down(): void
+    {
+        DB::statement('DROP INDEX IF EXISTS payment_records_bill_idempotency_unique');
+        DB::statement('DROP INDEX IF EXISTS settlement_adjustments_bill_idempotency_unique');
+
+        Schema::table('payment_records', function (Blueprint $table): void {
+            $table->index(['settlement_bill_id', 'idempotency_key']);
+        });
+
+        Schema::table('settlement_adjustments', function (Blueprint $table): void {
+            $table->index(['original_bill_id', 'idempotency_key']);
+            $table->dropConstrainedForeignId('result_bill_id');
+        });
+    }
+};