feat: Sprint 1 remainder — Postgres store, Docker client, warm pool, sessions route

- PostgresUserStore (Queryable) + migration + env-gated wiring
- DockerodeClient behind injected DockerEngine
- ImageManager, WarmPool, Dockerfile.wordpress, docker-compose
- POST /sessions spins up sandbox (503 when unconfigured)
- ADRs 0014-0015; 68 unit tests green

infrastructure/docker/Dockerfile.wordpress

@@ -0,0 +1,4 @@
+FROM wordpress:6.7-php8.2
+
+RUN curl -fsSL https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar -o /usr/local/bin/wp \
+    && chmod +x /usr/local/bin/wp

infrastructure/docker/docker-compose.yml

@@ -0,0 +1,30 @@
+services:
+  db:
+    image: mysql:8.0
+    environment:
+      MYSQL_DATABASE: wordpress
+      MYSQL_USER: wordpress
+      MYSQL_PASSWORD: wordpress
+      MYSQL_ROOT_PASSWORD: root
+    volumes:
+      - db_data:/var/lib/mysql
+
+  wordpress:
+    build:
+      context: .
+      dockerfile: Dockerfile.wordpress
+    depends_on:
+      - db
+    ports:
+      - "8080:80"
+    environment:
+      WORDPRESS_DB_HOST: db
+      WORDPRESS_DB_USER: wordpress
+      WORDPRESS_DB_PASSWORD: wordpress
+      WORDPRESS_DB_NAME: wordpress
+    volumes:
+      - wp_data:/var/www/html
+
+volumes:
+  db_data:
+  wp_data: