Fix db service network alias for backup.sh pg_dump connection

The backup.sh script hardcodes 'pg_dump -h dragonsstash-db' (matching
production), but the db service lacked a network alias for that hostname.
Added network alias to the backend network so the backup service can
successfully connect using the hardcoded hostname.

Verified: both 'db' and 'dragonsstash-db' resolve from the backend network.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.yml

@@ -145,8 +145,10 @@ services:
         limits:
           memory: 1G
     networks:
-      - frontend
+      frontend: {}
-      - backend
+      backend:
+        aliases:
+          - dragonsstash-db
 
 volumes:
   postgres_data: