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

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

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+-- ===========
+--  Down Migration
+-- ===========
+
+-- Drop triggers (must be dropped before their tables)
+DROP TRIGGER IF EXISTS update_routes_updated_at ON routes;
+DROP TRIGGER IF EXISTS update_projects_updated_at ON projects;
+DROP TRIGGER IF EXISTS update_workspaces_updated_at ON workspaces;
+DROP TRIGGER IF EXISTS update_plans_updated_at ON plans;
+DROP TRIGGER IF EXISTS update_users_updated_at ON users;
+
+-- Drop the trigger function (after all triggers are gone)
+DROP FUNCTION IF EXISTS update_updated_at_column();
+
+-- Drop indexes
+DROP INDEX IF EXISTS idx_routes_project_id;
+DROP INDEX IF EXISTS idx_projects_workspace_id;
+DROP INDEX IF EXISTS idx_workspaces_owner_id;
+
+-- Drop tables in reverse dependency order
+-- Product Logic
+DROP TABLE IF EXISTS routes;
+DROP TABLE IF EXISTS projects;
+
+-- RBAC
+DROP TABLE IF EXISTS role_permissions;
+DROP TABLE IF EXISTS permissions;
+DROP TABLE IF EXISTS roles;
+
+-- Tenancy
+DROP TABLE IF EXISTS workspace_members;
+DROP TABLE IF EXISTS workspaces;
+
+-- Users & Plans
+DROP TABLE IF EXISTS plans;
+DROP TABLE IF EXISTS users;