feat(infra): PostgreSQL16 compose（宿主5434）

Co-Authored-By: Claude Code <noreply@anthropic.com>

.env.example

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+# 复制为 .env 使用；.env 已被 gitignore
+DATABASE_URL=postgres://siteforge:siteforge_dev@localhost:5434/siteforge

infra/docker-compose.yml

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+# infra/docker-compose.yml
+services:
+  postgres:
+    image: postgres:16
+    container_name: sf-postgres
+    environment:
+      POSTGRES_USER: siteforge
+      POSTGRES_PASSWORD: siteforge_dev
+      POSTGRES_DB: siteforge
+    ports: ["5434:5432"]
+    volumes: [sf_pgdata:/var/lib/postgresql/data]
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U siteforge"]
+      interval: 5s
+      timeout: 3s
+      retries: 10
+volumes:
+  sf_pgdata: