feat: implement attachment database layer

- Add AttachmentInterface to main Datastore interface
- Implement CRUD operations for attachments following existing patterns
- Add proper SQL marshalling/unmarshalling with HEX/UNHEX for binary IDs
- Include soft deletion and proper indexing support

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

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

core/model/db/db.go

@@ -15,6 +15,7 @@ type Datastore interface {
 	OrgInterface
 	AccountInterface
 	TransactionInterface
+	AttachmentInterface
 	PriceInterface
 	SessionInterface
 	ApiKeyInterface