feat(env): add INITIAL_ADMIN_EMAIL to .env.example

- Include INITIAL_ADMIN_EMAIL in the environment variables example file
- Explain its purpose and usage for setting up the initial administrator account
- Clarify that it's only used on the first startup and won't change existing admin email

Author: fulleni

.env.example

@@ -33,3 +33,10 @@
 # Defaults to "https://api.sendgrid.com" if not set.
 # Use "https://api.eu.sendgrid.com" for EU-based accounts.
 # SENDGRID_API_URL="https://api.sendgrid.com"
+
+# REQUIRED FOR FIRST RUN: The email address for the initial administrator account.
+# This value is used ONLY on the first startup to create the first admin user.
+# If a user with this email already exists, nothing will happen.
+# Changing this value after the first run WILL NOT change the existing admin's email.
+# To create a new admin, you must do so from the dashboard after logging in.
+# INITIAL_ADMIN_EMAIL="admin@example.com"