feat: add attachment model type definitions

- Add core attachment type with metadata fields for transaction files
- Add GORM model for attachment with proper relationships
- Include file information, upload timestamps, and soft deletion support

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

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

core/model/types/attachment.go

@@ -0,0 +1,20 @@
+package types
+
+import (
+	"time"
+)
+
+type Attachment struct {
+	Id           string    `json:"id"`
+	TransactionId string    `json:"transactionId"`
+	OrgId        string    `json:"orgId"`
+	UserId       string    `json:"userId"`
+	FileName     string    `json:"fileName"`
+	OriginalName string    `json:"originalName"`
+	ContentType  string    `json:"contentType"`
+	FileSize     int64     `json:"fileSize"`
+	FilePath     string    `json:"filePath"`
+	Description  string    `json:"description"`
+	Uploaded     time.Time `json:"uploaded"`
+	Deleted      bool      `json:"deleted"`
+}