chore: restructure project into backend and frontend folders

- Move all frontend code to frontend/ directory
- Add backend/ with Go project structure (cmd, internal, pkg)
- Add docker-compose.yml for orchestration

backend/internal/db/migrations/000002_create_users_table.up.sql

@@ -0,0 +1,6 @@
+CREATE TABLE users (
+    id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+    username VARCHAR(50) NOT NULL UNIQUE,
+    password_hash VARCHAR(255) NOT NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);