feat: implement attachment business logic layer

- Add AttachmentInterface to main model interface
- Implement attachment CRUD operations with permission checking
- Add GetTransaction method for secure attachment access validation
- Add accountsContainReadAccess for permission verification
- Ensure users can only access attachments for authorized transactions

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/model/attachment.go

@@ -0,0 +1,163 @@
+package model
+
+import (
+	"errors"
+	"time"
+
+	"github.com/openaccounting/oa-server/core/model/types"
+)
+
+type AttachmentInterface interface {
+	CreateAttachment(*types.Attachment) (*types.Attachment, error)
+	GetAttachmentsByTransaction(string, string, string) ([]*types.Attachment, error)
+	GetAttachment(string, string, string, string) (*types.Attachment, error)
+	DeleteAttachment(string, string, string, string) error
+}
+
+func (model *Model) CreateAttachment(attachment *types.Attachment) (*types.Attachment, error) {
+	if attachment.Id == "" {
+		return nil, errors.New("attachment ID required")
+	}
+
+	if attachment.TransactionId == "" {
+		return nil, errors.New("transaction ID required")
+	}
+
+	if attachment.OrgId == "" {
+		return nil, errors.New("organization ID required")
+	}
+
+	if attachment.UserId == "" {
+		return nil, errors.New("user ID required")
+	}
+
+	if attachment.FileName == "" {
+		return nil, errors.New("file name required")
+	}
+
+	if attachment.FilePath == "" {
+		return nil, errors.New("file path required")
+	}
+
+	// Set upload timestamp
+	attachment.Uploaded = time.Now()
+	attachment.Deleted = false
+
+	// Save to database
+	err := model.db.InsertAttachment(attachment)
+	if err != nil {
+		return nil, err
+	}
+
+	return attachment, nil
+}
+
+func (model *Model) GetAttachmentsByTransaction(transactionId, orgId, userId string) ([]*types.Attachment, error) {
+	if transactionId == "" {
+		return nil, errors.New("transaction ID required")
+	}
+
+	if orgId == "" {
+		return nil, errors.New("organization ID required")
+	}
+
+	if userId == "" {
+		return nil, errors.New("user ID required")
+	}
+
+	// First verify the user has access to the transaction
+	tx, err := model.GetTransaction(transactionId, orgId, userId)
+	if err != nil {
+		return nil, err
+	}
+	if tx == nil {
+		return nil, errors.New("transaction not found or access denied")
+	}
+
+	// Get attachments for the transaction
+	attachments, err := model.db.GetAttachmentsByTransaction(transactionId, orgId)
+	if err != nil {
+		return nil, err
+	}
+
+	return attachments, nil
+}
+
+func (model *Model) GetAttachment(attachmentId, transactionId, orgId, userId string) (*types.Attachment, error) {
+	if attachmentId == "" {
+		return nil, errors.New("attachment ID required")
+	}
+
+	if transactionId == "" {
+		return nil, errors.New("transaction ID required")
+	}
+
+	if orgId == "" {
+		return nil, errors.New("organization ID required")
+	}
+
+	if userId == "" {
+		return nil, errors.New("user ID required")
+	}
+
+	// First verify the user has access to the transaction
+	tx, err := model.GetTransaction(transactionId, orgId, userId)
+	if err != nil {
+		return nil, err
+	}
+	if tx == nil {
+		return nil, errors.New("transaction not found or access denied")
+	}
+
+	// Get the attachment
+	attachment, err := model.db.GetAttachment(attachmentId, transactionId, orgId)
+	if err != nil {
+		return nil, err
+	}
+
+	return attachment, nil
+}
+
+func (model *Model) DeleteAttachment(attachmentId, transactionId, orgId, userId string) error {
+	if attachmentId == "" {
+		return errors.New("attachment ID required")
+	}
+
+	if transactionId == "" {
+		return errors.New("transaction ID required")
+	}
+
+	if orgId == "" {
+		return errors.New("organization ID required")
+	}
+
+	if userId == "" {
+		return errors.New("user ID required")
+	}
+
+	// First verify the user has access to the transaction
+	tx, err := model.GetTransaction(transactionId, orgId, userId)
+	if err != nil {
+		return err
+	}
+	if tx == nil {
+		return errors.New("transaction not found or access denied")
+	}
+
+	// Verify the attachment exists and belongs to the transaction
+	attachment, err := model.db.GetAttachment(attachmentId, transactionId, orgId)
+	if err != nil {
+		return err
+	}
+	if attachment == nil {
+		return errors.New("attachment not found")
+	}
+
+	// Soft delete the attachment
+	err = model.db.DeleteAttachment(attachmentId, transactionId, orgId)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}