feat(billing): 完成异步结算与请求执行闭环

统一任务成功、Attempt 与结算 Outbox 的事务边界，增加多实例安全结算、人工复核、请求幂等与执行租约。钱包决策使用九位精确金额，并通过审计保护约束保留流水事实；同时补充管理接口、指标与 PostgreSQL 集成测试。

apps/api/internal/httpapi/billing_settlement_handlers_test.go

@@ -0,0 +1,21 @@
+package httpapi
+
+import (
+	"net/http/httptest"
+	"testing"
+)
+
+func TestSingleIdempotencyKey(t *testing.T) {
+	t.Parallel()
+
+	request := httptest.NewRequest("POST", "/", nil)
+	request.Header.Add("Idempotency-Key", "retry-1")
+	if got, ok := singleIdempotencyKey(request); !ok || got != "retry-1" {
+		t.Fatalf("got key=%q ok=%v", got, ok)
+	}
+
+	request.Header.Add("Idempotency-Key", "retry-2")
+	if _, ok := singleIdempotencyKey(request); ok {
+		t.Fatal("multiple Idempotency-Key values must be rejected")
+	}
+}