fix: remove all default credentials (#2)

- Gateway: admin user seeded from ADMIN_USERNAME/ADMIN_PASSWORD env vars
  (no more hardcoded admin/admin). Warns if not set.
- Trips: USERNAME/PASSWORD env vars no longer default to admin/admin.
  Warns if not set.
- Fitness: user seed requires USER{n}_USERNAME/PASSWORD env vars.
  No more "changeme" fallback. Skips seed if not set.
- /api/auth/register remains disabled (403)

Closes #2

docker-compose.yml

@@ -30,6 +30,9 @@ services:
       - /media/yusiboyz/Media/Scripts/booklore/booklore/books:/booklore-books:ro
       - /media/yusiboyz/Media/Scripts/shelfmark/books:/bookdrop:ro
     environment:
+      - ADMIN_USERNAME=${ADMIN_USERNAME}
+      - ADMIN_PASSWORD=${ADMIN_PASSWORD}
+      - ADMIN_DISPLAY_NAME=${ADMIN_DISPLAY_NAME:-Admin}
       - PORT=8100
       - TRIPS_BACKEND_URL=http://trips-service:8087
       - FITNESS_BACKEND_URL=http://fitness-service:8095