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/model.go

@@ -20,11 +20,13 @@ type Interface interface {
 	OrgInterface
 	AccountInterface
 	TransactionInterface
+	AttachmentInterface
 	PriceInterface
 	SessionInterface
 	ApiKeyInterface
 	SystemHealthInteface
 	BudgetInterface
+	GetTransaction(string, string, string) (*types.Transaction, error)
 }
 
 func NewModel(db db.Datastore, bcrypt util.Bcrypt, config types.Config) *Model {