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/task_idempotency_test.go

@@ -0,0 +1,31 @@
+package httpapi
+
+import (
+	"net/http/httptest"
+	"testing"
+)
+
+func TestOptionalTaskIdempotencyKeyRejectsMultipleValues(t *testing.T) {
+	t.Parallel()
+	request := httptest.NewRequest("POST", "/", nil)
+	if _, present, err := optionalTaskIdempotencyKey(request); err != nil || present {
+		t.Fatalf("missing key present=%v err=%v", present, err)
+	}
+	request.Header.Add("Idempotency-Key", "one")
+	request.Header.Add("Idempotency-Key", "two")
+	if _, _, err := optionalTaskIdempotencyKey(request); err == nil {
+		t.Fatal("multiple keys must be rejected")
+	}
+}
+
+func TestTaskIdempotencyRequestHashIsCanonical(t *testing.T) {
+	t.Parallel()
+	first := map[string]any{"model": "m", "n": float64(1), "nested": map[string]any{"b": true, "a": "x"}}
+	second := map[string]any{"nested": map[string]any{"a": "x", "b": true}, "n": float64(1), "model": "m"}
+	if taskIdempotencyRequestHash("images.generations", false, false, first) != taskIdempotencyRequestHash("images.generations", false, false, second) {
+		t.Fatal("equivalent JSON objects must have the same request hash")
+	}
+	if taskIdempotencyRequestHash("images.generations", true, false, first) == taskIdempotencyRequestHash("images.generations", false, false, first) {
+		t.Fatal("async response semantics must be part of the request hash")
+	}
+}