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/normalize_folder_paths.sql

@@ -0,0 +1,15 @@
+-- Normalize existing folder paths in user_ebook_folders table
+-- This script will clean up inconsistent path formatting
+
+UPDATE user_ebook_folders 
+SET folder_path = REPLACE(REPLACE(folder_path, '\\', '/'), '//', '/')
+WHERE folder_path LIKE '%\\%' OR folder_path LIKE '%//%';
+
+-- Remove trailing slashes from non-root paths
+UPDATE user_ebook_folders 
+SET folder_path = CASE 
+    WHEN folder_path = '/' THEN '/'
+    WHEN RIGHT(folder_path, 1) = '/' THEN LEFT(folder_path, LENGTH(folder_path) - 1)
+    ELSE folder_path
+END
+WHERE folder_path != '/' AND RIGHT(folder_path, 1) = '/';