feat(backend/repositories): add get tags by file method

refactor(backend/domain): start switching from pgx types

backend/internal/domain/entity.go

@@ -20,9 +20,9 @@ type MIME struct {
 
 type (
 	CategoryCore struct {
-		ID    string      `json:"id"`
-		Name  string      `json:"name"`
-		Color pgtype.Text `json:"color"`
+		ID    *string `json:"id"`
+		Name  *string `json:"name"`
+		Color *string `json:"color"`
 	}
 	CategoryItem struct {
 		CategoryCore

backend/internal/domain/repositories.go

@@ -13,4 +13,5 @@ type FileRepository interface {
 	Add(ctx context.Context, user_id int, name, mime string, datetime time.Time, notes string, metadata json.RawMessage) (file FileCore, domainErr *DomainError)
 	Update(ctx context.Context, user_id int, file_id string, updates map[string]interface{}) (domainErr *DomainError)
 	Delete(ctx context.Context, user_id int, file_id string) (domainErr *DomainError)
+	GetTags(ctx context.Context, user_id int, file_id string) (tags []TagItem, domainErr *DomainError)
 }