Use bind mount for prod database storage instead of named volume

Store PostgreSQL data at ./data/postgres relative to the compose file
so persistent data lives on the Unraid disk at
/mnt/user/appdata/nuzlocke-tracker/data/postgres.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -26,7 +26,7 @@ services:
       - POSTGRES_PASSWORD=${POSTGRES_PASSWORD}
       - POSTGRES_DB=nuzlocke
     volumes:
-      - prod_postgres_data:/var/lib/postgresql/data
+      - ./data/postgres:/var/lib/postgresql/data
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U postgres"]
       interval: 5s
@@ -34,5 +34,3 @@ services:
       retries: 5
     restart: unless-stopped
 
-volumes:
-  prod_postgres_data: