Add webmention sender endpoint integration

.env.example

@@ -9,3 +9,13 @@ PODROLL_EPISODES_URL=
 # FreshRSS OPML export URL for source sync
 # Example: https://freshrss.example/api/query.php?user=USER&t=TOKEN&f=opml
 PODROLL_OPML_URL=
+
+# Optional webmention sender endpoint settings
+# Default mount path in indiekit.config.mjs is /webmention-sender
+WEBMENTION_SENDER_MOUNT_PATH=/webmention-sender
+
+# Endpoint discovery timeout (milliseconds)
+WEBMENTION_SENDER_TIMEOUT=10000
+
+# User-Agent used for target endpoint discovery and sends
+WEBMENTION_SENDER_USER_AGENT=