chore: rename postgres service to db

docs/operations.md

@@ -5,7 +5,7 @@
 ```bash
 docker compose up --build -d
 docker compose ps
-docker compose logs -f proxy backend frontend postgres
+docker compose logs -f proxy backend frontend db
 docker compose down
 ```
 
@@ -33,7 +33,7 @@ Back up the database and media at the same logical point in time:
 ```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
 ```