♻️ Simplify optional environment configuration (#2417)

## AI Disclaimer

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

.env

@@ -10,17 +10,12 @@ ENVIRONMENT=local
 
 PROJECT_NAME="Full Stack FastAPI Project"
 
-# Backend
-# Additional allowed CORS origins, separated by commas
-BACKEND_CORS_ORIGINS=
 SECRET_KEY=changethis
 FIRST_SUPERUSER=admin@example.com
 FIRST_SUPERUSER_PASSWORD=changethis
 
 # Emails
 SMTP_HOST=localhost
-SMTP_USER=
-SMTP_PASSWORD=
 EMAILS_FROM_EMAIL=info@example.com
 SMTP_TLS=False
 SMTP_PORT=1025
@@ -30,5 +25,3 @@ POSTGRES_SERVER=localhost
 POSTGRES_DB=app
 POSTGRES_USER=postgres
 POSTGRES_PASSWORD=changethis
-
-SENTRY_DSN=