fix(db): handle explicit null values in update requests

Introduce a `NullString` type to distinguish between JSON `null` and
absent fields in `UpdateDrawingRequest`. This ensures that when a
field is explicitly set to `null` in a request, the database can
correctly process the update.

Additionally, refactor the folder order migration to include proper
`goose` up/down instructions.

internal/postgres/migrations/00003_folder_order.sql

@@ -1,2 +1,7 @@
-ALTER TABLE workspace_folders ADD COLUMN IF NOT EXISTS sort_order INTEGER NOT NULL DEFAULT 0;
-CREATE INDEX IF NOT EXISTS idx_workspace_folders_sort_order ON workspace_folders(team_id, sort_order);
+-- +goose Up
+ALTER TABLE workspace_folders ADD COLUMN sort_order INTEGER NOT NULL DEFAULT 0;
+CREATE INDEX idx_workspace_folders_sort_order ON workspace_folders(team_id, sort_order);
+
+-- +goose Down
+DROP INDEX IF EXISTS idx_workspace_folders_sort_order;
+ALTER TABLE workspace_folders DROP COLUMN IF EXISTS sort_order;

workspace/store.go

@@ -40,9 +40,30 @@ type CreateDrawingRequest struct {
 	Snapshot    json.RawMessage `json:"snapshot"`
 }
 
+// NullString distinguishes JSON null (Valid=true, Value=nil) from absent field (Valid=false).
+type NullString struct {
+	Value *string
+	Valid bool
+}
+
+func (n *NullString) UnmarshalJSON(data []byte) error {
+	if string(data) == "null" {
+		n.Value = nil
+		n.Valid = true
+		return nil
+	}
+	var s string
+	if err := json.Unmarshal(data, &s); err != nil {
+		return err
+	}
+	n.Value = &s
+	n.Valid = true
+	return nil
+}
+
 type UpdateDrawingRequest struct {
-	FolderID    *string `json:"folder_id"`
-	ProjectID   *string `json:"project_id"`
+	FolderID    NullString `json:"folder_id"`
+	ProjectID   NullString `json:"project_id"`
 	Title       *string    `json:"title"`
 	Description *string    `json:"description"`
 	Visibility  *string    `json:"visibility"`
@@ -621,11 +642,11 @@ func (s *Store) UpdateDrawing(ctx context.Context, userID, drawingID string, req
 		}
 		current.Visibility = *req.Visibility
 	}
-	if req.FolderID != nil {
-		current.FolderID = req.FolderID
+	if req.FolderID.Valid {
+		current.FolderID = req.FolderID.Value
 	}
-	if req.ProjectID != nil {
-		current.ProjectID = req.ProjectID
+	if req.ProjectID.Valid {
+		current.ProjectID = req.ProjectID.Value
 	}
 	now := time.Now().UTC()
 	_, err = s.db.ExecContext(ctx, `UPDATE workspace_drawings