fix(api): add custom headers to email fallback SMTP (#3507)

deploy/self-hosting/.env.example

@@ -334,6 +334,9 @@ FLUXER_EMAIL_SMTP_TLS_MODE=
 #FLUXER_EMAIL_FALLBACK_SMTP_USERNAME=
 #FLUXER_EMAIL_FALLBACK_SMTP_PASSWORD=
 #FLUXER_EMAIL_FALLBACK_SMTP_TLS_MODE=
+# Extra headers on every fallback message, as a JSON object of header name to
+# string value. Use single quotes so the shell keeps the JSON intact.
+#FLUXER_EMAIL_FALLBACK_SMTP_HEADERS='{"X-Example-Tracking":"off"}'
 #FLUXER_EMAIL_WEBHOOK_SECRET=
 
 FLUXER_DISCOVERY_ENABLED=true