docs: add API_URL environment variable to .env.example files

Document the API_URL environment variable that is used for constructing
URLs for assets (logos, images) in email notifications. Without this
setting, the system defaults to http://localhost:3001 which causes
broken images in production emails.

Added to both root and backend .env.example files with clear
documentation about its purpose and importance.

.env.example

@@ -39,6 +39,11 @@ [email protected]
 FRONTEND_URL=https://yourdomain.com
 ADMIN_URL=https://yourdomain.com
 
+# API URL for email assets (logos, images in notification emails)
+# This must be the publicly accessible URL where email recipients can load images.
+# If not set, defaults to http://localhost:3001 which will show broken images in emails.
+API_URL=https://yourdomain.com/api
+
 # Frontend API base
 # For pre-built images and production behind a reverse proxy, keep '/api'.
 # If you rebuild the frontend yourself, you may set a full URL at build time.

backend/.env.example

@@ -14,6 +14,11 @@ ADMIN_URL=https://photos.example.com
 FRONTEND_URL=https://photos.example.com
 BACKEND_URL=https://photos.example.com  # Or https://api.photos.example.com if separate
 
+# API URL for email assets (logos, images in emails)
+# This must be the publicly accessible URL where recipients can load images
+# If not set, defaults to http://localhost:3001 which will break images in production emails
+API_URL=https://photos.example.com/api
+
 # Database Configuration
 DATABASE_CLIENT=pg
 DB_HOST=localhost