refactor(config): rename initial admin email to override admin email

- Update environment variable name from INITIAL_ADMIN_EMAIL to OVERRIDE_ADMIN_EMAIL
- Modify documentation to reflect that this variable is used to set or replace the single administrator account on startup

Author: fulleni

lib/src/config/environment_config.dart

@@ -134,9 +134,9 @@ abstract final class EnvironmentConfig {
   /// Returns `null` if the `SENDGRID_API_URL` is not set.
   static String? get sendGridApiUrl => _env['SENDGRID_API_URL'];
 
-  /// Retrieves the initial admin email from the environment, if provided.
+  /// Retrieves the override admin email from the environment, if provided.
   ///
-  /// This is used for seeding the first administrator account on startup.
-  /// Returns `null` if the `INITIAL_ADMIN_EMAIL` is not set.
-  static String? get initialAdminEmail => _env['INITIAL_ADMIN_EMAIL'];
+  /// This is used to set or replace the single administrator account on startup.
+  /// Returns `null` if the `OVERRIDE_ADMIN_EMAIL` is not set.
+  static String? get overrideAdminEmail => _env['OVERRIDE_ADMIN_EMAIL'];
 }