refactor: reorganize project structure and update configurations

- Move migrations/ to database/schema/ for clarity on database schema definitions
- Move sqlc.yaml to internal/database/ to group with database code
- Move static/ to cmd/server/static/ to co-locate with server
- Update all configuration files and documentation
- Follow Go project conventions for better organization

internal/database/queries/queries.sql

@@ -131,8 +131,8 @@ INSERT INTO user_ebook_folders (user_id, folder_path) VALUES ($1, $2) RETURNING
 -- name: GetUserEbookFolders :many
 SELECT * FROM user_ebook_folders WHERE user_id = $1 ORDER BY created_at;
 
--- name: DeleteUserEbookFolder :exec
-DELETE FROM user_ebook_folders WHERE user_id = $1 AND folder_path = $2;
+-- name: DeleteUserEbookFolder :one
+DELETE FROM user_ebook_folders WHERE user_id = $1 AND folder_path = $2 RETURNING *;
 
 -- name: GetEbookByFilePath :one
 SELECT * FROM ebooks WHERE file_path = $1;