feat: DB-driven whitelist with is_admin, auto-register on /start, admin commands (/adduser /removeuser /users)

supabase/migrations/01-init.sql

@@ -10,6 +10,7 @@ CREATE TABLE IF NOT EXISTS users (
     telegram_id bigint UNIQUE NOT NULL,
     username    text,
     first_name  text,
+    is_admin    boolean NOT NULL DEFAULT false,
     is_active   boolean NOT NULL DEFAULT true,
     created_at  timestamptz NOT NULL DEFAULT now()
 );