add is_folder notion

backend/internal/db/migrations/004_folders.down.sql

@@ -0,0 +1,2 @@
+-- VaultDrop 003: tags table
+ALTER TABLE files DROP COLUMN IF EXISTS is_folder;

backend/internal/db/migrations/004_folders.up.sql

@@ -0,0 +1 @@
+ALTER TABLE files ADD COLUMN is_folder BOOLEAN NOT NULL DEFAULT false;