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/runner/billing_settlement_worker_test.go

@@ -0,0 +1,40 @@
+package runner
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/easyai/easyai-ai-gateway/apps/api/internal/store"
+)
+
+func TestBillingSettlementRetryDelay(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		attempt int
+		want    time.Duration
+	}{
+		{attempt: 1, want: time.Second},
+		{attempt: 2, want: 2 * time.Second},
+		{attempt: 10, want: 512 * time.Second},
+		{attempt: 11, want: 15 * time.Minute},
+		{attempt: 20, want: 15 * time.Minute},
+	}
+	for _, test := range tests {
+		if got := billingSettlementRetryDelay(test.attempt); got != test.want {
+			t.Fatalf("attempt %d: got %s, want %s", test.attempt, got, test.want)
+		}
+	}
+}
+
+func TestBillingSettlementErrorCode(t *testing.T) {
+	t.Parallel()
+
+	if got := billingSettlementErrorCode(store.ErrInsufficientWalletBalance); got != "insufficient_balance" {
+		t.Fatalf("got %q", got)
+	}
+	if got := billingSettlementErrorCode(errors.New("boom")); got != "settlement_failed" {
+		t.Fatalf("got %q", got)
+	}
+}