Use docker-manager's own Postgres role

Infrastructure now gives each project its own role, owner of its
database only, instead of handing out the superuser. DATABASE_URL uses
docker-manager's (`dockermanager`), whose password matches
Infrastructure's DOCKERMANAGER_DB_PASSWORD.

backend/.env.example

@@ -3,9 +3,10 @@ PORT=8090
 CORS_ALLOWED_ORIGINS=http://localhost:5173
 
 # Postgres, Zitadel and SeaweedFS come from the shared ../../Infrastructure
-# stack (`make dev-infra`), not from a container in this repo. Credentials
-# must match Infrastructure's own .env.
-DATABASE_URL=postgres://postgres:postgres@host.docker.internal:5432/dockermanager_db?sslmode=disable
+# stack (`make dev-infra`), not from a container in this repo. docker-manager
+# has its own role there, owner of dockermanager_db only; its password must
+# match Infrastructure's DOCKERMANAGER_DB_PASSWORD.
+DATABASE_URL=postgres://dockermanager:dockermanager@host.docker.internal:5432/dockermanager_db?sslmode=disable
 
 # Not wired into the backend yet (see README). These point at Infrastructure's
 # Zitadel for when OIDC login is implemented.