chore(env): whitelist DB_APP_ROLE and document it beside the database settings

.env.example

@@ -38,6 +38,12 @@
 #   echo "POSTGRES_PASSWORD=$(openssl rand -hex 32)" >> .env
 # Left commented here so that append does not create a duplicate key. Uncomment
 # and set it directly instead if you prefer to edit the file by hand.
+
+# The container runs the app as `healthlog_app`, a database role it creates on
+# every start that reads and writes rows but cannot change the schema. The
+# migrations keep the DATABASE_URL user. "off" keeps the app on that user, if
+# you manage database roles yourself (docs/self-hosting/database-role.md).
+# DB_APP_ROLE="off"
 # POSTGRES_PASSWORD="generate-with-openssl-rand-hex-32"