chore: rename postgres service to db

README.md

@@ -12,7 +12,7 @@ stores content, and Docker Compose supplies a reproducible local environment.
   Italian fallback content for development when the CMS is unavailable.
 - `nginx/`: reverse-proxy virtual host for `azionelab.org`, routing public requests to
   Astro or Wagtail.
-- `postgres`: persistent CMS database.
+- `db`: persistent PostgreSQL database.
 - Docker volumes: `postgres_data` for the database and `media_data` for uploads.
 
 See [the architecture document](docs/architecture.md),
@@ -91,11 +91,15 @@ The frontend reads `PUBLIC_CMS_API_URL`; inside Compose it defaults to
 public reverse-proxy address. Change both values for another environment.
 The standard published-page endpoint is also available at `/api/v2/pages/`.
 
+The PostgreSQL service is named `db`. Existing `.env` files should replace `@postgres:`
+with `@db:` in `DATABASE_URL`; a compatibility network alias keeps older local values
+working during the transition.
+
 ## Useful commands
 
 ```bash
 # Follow application logs
-docker compose logs -f postgres backend frontend proxy
+docker compose logs -f db backend frontend proxy
 
 # Run database migrations
 docker compose exec backend python manage.py migrate
@@ -137,7 +141,7 @@ Database rows and uploaded files must be backed up together:
 ```bash
 mkdir -p backups/media
 docker compose stop frontend backend
-docker compose exec -T postgres sh -c 'pg_dump -U "$POSTGRES_USER" "$POSTGRES_DB"' > backups/database.sql
+docker compose exec -T db sh -c 'pg_dump -U "$POSTGRES_USER" "$POSTGRES_DB"' > backups/database.sql
 docker compose cp backend:/app/media/. backups/media/
 docker compose start backend frontend
 ```