Convert backend from submodule to regular directory

Remove submodule tracking; backend is now a plain directory in the repo.
Also update deploy workflow: remove --recurse-submodules.

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

backend/stop.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+PORT="${LISTEN_ADDR:-:8081}"
+PORT="${PORT#:}"
+
+PID=$(lsof -ti:$PORT)
+if [ -z "$PID" ]; then
+  echo "Server is not running on port $PORT"
+  exit 0
+fi
+
+kill $PID
+echo "Server stopped (port $PORT, pid $PID)"