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

database/schema/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;