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/repository/settlement.go

@@ -63,7 +63,7 @@ func (r *SettlementRepository) GetByID(ctx context.Context, supplierID, id int64
 	`
 
 	s := &domain.Settlement{}
-	var paidAt pgx.NullTime
+	var paidAt *time.Time
 	err := r.pool.QueryRow(ctx, query, id, supplierID).Scan(
 		&s.ID, &s.SettlementNo, &s.SupplierID, &s.TotalAmount, &s.FeeAmount, &s.NetAmount,
 		&s.Status, &s.PaymentMethod, &s.PaymentAccount,
@@ -79,8 +79,8 @@ func (r *SettlementRepository) GetByID(ctx context.Context, supplierID, id int64
 		return nil, fmt.Errorf("failed to get settlement: %w", err)
 	}
 
-	if paidAt.Valid {
-		s.PaidAt = &paidAt.Time
+	if paidAt != nil {
+		s.PaidAt = paidAt
 	}
 
 	return s, nil