[init] Add initial database migration files for PostgreSQL, SQLite, and MySQL

api/migrations/mysql/001_init.down.sql

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+-- Down migration for 001_init
+
+START TRANSACTION;
+
+-- Disable FK checks to avoid dependency issues during drops
+SET FOREIGN_KEY_CHECKS = 0;
+
+-- Drop indexes (will be removed automatically with tables, but explicit drops keep intent clear)
+DROP INDEX idx_routes_project_id ON routes;
+DROP INDEX idx_projects_workspace_id ON projects;
+DROP INDEX idx_workspaces_owner_id ON workspaces;
+
+-- Drop tables in reverse dependency order
+DROP TABLE IF EXISTS role_permissions;
+DROP TABLE IF EXISTS workspace_members;
+DROP TABLE IF EXISTS routes;
+DROP TABLE IF EXISTS projects;
+DROP TABLE IF EXISTS permissions;
+DROP TABLE IF EXISTS roles;
+DROP TABLE IF EXISTS workspaces;
+DROP TABLE IF EXISTS plans;
+DROP TABLE IF EXISTS users;
+
+-- Re-enable FK checks
+SET FOREIGN_KEY_CHECKS = 1;
+
+COMMIT;