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

.env.production.example

@@ -526,6 +526,14 @@ API_TOKEN_HMAC_KEY="<openssl rand -hex 32>"
 # Behind such a pooler, set this to 1, true or yes to send no session
 # settings at all; the pooler's or the database's own defaults then apply.
 # DATABASE_SESSION_OPTIONS_DISABLED="1"
+#
+# The app runs as `healthlog_app`, a database role the container creates on
+# every start: it reads and writes rows but cannot alter or drop tables or
+# create roles. Migrations and restores still use the user in DATABASE_URL.
+# Where that user may not create roles (a managed database), the start log
+# says so and the app uses DATABASE_URL as given. Set "off" if you manage
+# the database roles yourself. See docs/self-hosting/database-role.md.
+# DB_APP_ROLE="off"
 
 
 # -----------------------------------------------------------------------------