fix(supply-api): 修复编译错误和测试问题

- 添加 ErrNotFound 和 ErrConcurrencyConflict 错误定义
- 修复 pgx.NullTime 替换为 *time.Time
- 修复 db.go 事务类型 (pgx.Tx vs pgxpool.Tx)
- 移除未使用的导入和变量
- 修复 NewSupplyAPI 调用参数
- 修复中间件链路 handler 类型问题
- 修复适配器类型引用 (storage.InMemoryAccountStore 等)
- 所有测试通过

Test: go test ./...

supply-api/internal/middleware/idempotency_test.go

@@ -54,7 +54,7 @@ func (r *MockIdempotencyRepository) AcquireLock(ctx context.Context, tenantID, o
 	record := &repository.IdempotencyRecord{
 		TenantID:       tenantID,
 		OperatorID:     operatorID,
-		APIPath:       apiPath,
+		APIPath:        apiPath,
 		IdempotencyKey: idempotencyKey,
 		RequestID:      "test-request-id",
 		PayloadHash:    "",