test: add self-contained integration environment

Separate schema migrations from environment-specific fixtures, run the full API workflow against a disposable PostgreSQL Testcontainer, and provide seeded development and UI test environments.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docker-compose.yml

@@ -20,7 +20,8 @@ services:
     environment:
       DATABASE_URL: postgres://go_guess:go_guess@db:5432/go_guess?sslmode=disable
       JWT_SECRET: local-development-secret-change-me-12345
-      FRONTEND_URL: http://localhost:5173
+      FRONTEND_URL: ${FRONTEND_URL:-http://localhost:5173}
+      APP_ENV: ${APP_ENV:-development}
     ports:
       - "${API_PORT:-8080}:8080"
     depends_on: