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_mode_test.go

@@ -0,0 +1,27 @@
+package runner
+
+import "testing"
+
+func TestNormalizedBillingEngineMode(t *testing.T) {
+	t.Parallel()
+	if got := normalizedBillingEngineMode(""); got != "observe" {
+		t.Fatalf("empty mode = %q", got)
+	}
+	if got := normalizedBillingEngineMode("ENFORCE"); got != "enforce" {
+		t.Fatalf("enforce mode = %q", got)
+	}
+	if got := normalizedBillingEngineMode("hold"); got != "hold" {
+		t.Fatalf("hold mode = %q", got)
+	}
+}
+
+func TestBillingItemsFixedTotalKeepsNineDecimalPlaces(t *testing.T) {
+	t.Parallel()
+	items := []any{
+		map[string]any{"amount": "0.000000001"},
+		map[string]any{"amount": float64(0.000000002)},
+	}
+	if got := billingItemsFixedTotal(items).String(); got != "0.000000003" {
+		t.Fatalf("total = %s", got)
+	}
+}