chore: use shared postgres for family wishlist

.env.example

@@ -1,12 +1,14 @@
 # ==========================================
 #  Database
 # ==========================================
-POSTGRES_USER=wishlist
-POSTGRES_PASSWORD=change_me
-POSTGRES_DB=family_wishlist
-# DATABASE_URL uses the docker-compose service name `postgres`.
-# When running backend outside docker against docker-postgres use localhost:5432.
-DATABASE_URL=postgresql://wishlist:change_me@postgres:5432/family_wishlist
+DB_HOST=postgres_budget
+DB_PORT=5432
+DB_NAME=db_family
+DB_USER=
+DB_PASSWORD=
+# Fill DATABASE_URL explicitly; .env files do not expand ${...} automatically for the app.
+# For local host-based development, point it to localhost:5432 instead of postgres_budget.
+DATABASE_URL=postgresql://<db_user>:<db_password>@postgres_budget:5432/db_family
 
 # ==========================================
 #  Users (two fixed accounts)