fix(start): kill node process on service stop to prevent orphaned port binding

Trap previously only killed the webmention poller, leaving the node process
orphaned on service stop. On restart, the new node instance would fail to bind
port 3000 (EADDRINUSE) causing 502s with clean logs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

start.example.sh

@@ -104,6 +104,6 @@ WEBMENTION_ORIGIN="${PUBLICATION_URL:-${SITE_URL:-}}"
 ) &
 POLLER_PID="$!"
 
-trap 'kill "${POLLER_PID}" 2>/dev/null || true' EXIT INT TERM
+trap 'kill "${INDIEKIT_PID}" "${POLLER_PID}" 2>/dev/null || true; wait "${INDIEKIT_PID}" 2>/dev/null || true' EXIT INT TERM
 
 wait "${INDIEKIT_PID}"