♻️ Simplify environment defaults (#2415)

## AI Disclaimer

Using Codex with gpt-5.6-sol. Reviewed manually.

.env

@@ -5,10 +5,6 @@ DOMAIN=localhost
 # To test the local Traefik config
 # DOMAIN=localhost.tiangolo.com
 
-# Public frontend URL, used by the backend to generate links in emails.
-# Deployments derive this from DOMAIN in compose.deploy.yml.
-FRONTEND_HOST=http://localhost:5173
-
 # Environment: local, staging, production
 ENVIRONMENT=local
 
@@ -31,7 +27,6 @@ SMTP_PORT=1025
 
 # Postgres
 POSTGRES_SERVER=localhost
-POSTGRES_PORT=5432
 POSTGRES_DB=app
 POSTGRES_USER=postgres
 POSTGRES_PASSWORD=changethis