docs(env): update documentation for conditional environment variables

- Change REQUIRED to CONDITIONALLY REQUIRED for Firebase and OneSignal variables
- Add clarification that the server will start without these variables
- Emphasize the importance of these variables for their respective notification services

Author: fulleni

.env.example

@@ -57,23 +57,28 @@
 # Defaults to 15 minutes if not specified.
 # COUNTRY_SERVICE_CACHE_MINUTES=15
 
-# REQUIRED: The Firebase Project ID for push notifications.
+# CONDITIONALLY REQUIRED: The Firebase Project ID for push notifications.
+# The server will start without this, but it is required to use Firebase for notifications.
 # This is part of your Firebase service account credentials.
 # FIREBASE_PROJECT_ID="your-firebase-project-id"
 
-# REQUIRED: The Firebase Client Email for push notifications.
+# CONDITIONALLY REQUIRED: The Firebase Client Email for push notifications.
+# The server will start without this, but it is required to use Firebase for notifications.
 # This is part of your Firebase service account credentials.
 # FIREBASE_CLIENT_EMAIL="your-firebase-client-email"
 
-# REQUIRED: The Firebase Private Key for push notifications.
+# CONDITIONALLY REQUIRED: The Firebase Private Key for push notifications.
+# The server will start without this, but it is required to use Firebase for notifications.
 # This is part of your Firebase service account credentials.
 # Ensure this is stored securely and correctly formatted (e.g., replace newlines with \n if needed for single-line env var).
 # FIREBASE_PRIVATE_KEY="your-firebase-private-key"
 
-# REQUIRED: The OneSignal App ID for push notifications.
+# CONDITIONALLY REQUIRED: The OneSignal App ID for push notifications.
+# The server will start without this, but it is required to use OneSignal for notifications.
 # This identifies your application within OneSignal.
 # ONESIGNAL_APP_ID="your-onesignal-app-id"
 
-# REQUIRED: The OneSignal REST API Key for server-side push notifications.
+# CONDITIONALLY REQUIRED: The OneSignal REST API Key for server-side push notifications.
+# The server will start without this, but it is required to use OneSignal for notifications.
 # This is used to authenticate with the OneSignal API.
 # ONESIGNAL_REST_API_KEY="your-onesignal-rest-api-key"