Add self-hosted Postgres docker-compose

- docker-compose.yml: Postgres 16 Alpine with named volume, healthcheck
- .env.example: POSTGRES_* and DATABASE_URL template
- .gitignore: exclude db_data/ volume

Starting fresh, no data migration. App still wired to Supabase;
DB is ready for migrations to be applied.

docker-compose.yml

@@ -0,0 +1,25 @@
+services:
+  db:
+    image: postgres:16-alpine
+    container_name: route_commerce_db
+    restart: unless-stopped
+    env_file:
+      - .env
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
+      POSTGRES_DB: ${POSTGRES_DB}
+    volumes:
+      - db_data:/var/lib/postgresql/data
+    ports:
+      - "127.0.0.1:5432:5432"
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB}"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+      start_period: 10s
+
+volumes:
+  db_data:
+    driver: local