feat: add database support for multiple user ebook folders

- Add ebook_folder_path column to users table (migration 004)
- Create user_ebook_folders table for multiple folders per user (migration 005)
- Remove old ebook_folder_path column from users table (migration 006)
- Add unique constraint to prevent duplicate folder paths per user

backend/migrations/006_drop_ebook_folder_path.up.sql

@@ -0,0 +1,2 @@
+-- Drop the old ebook_folder_path column from users table
+ALTER TABLE users DROP COLUMN IF EXISTS ebook_folder_path;