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/start.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+set -e
+
+export DATABASE_URL="${DATABASE_URL:-postgres://ralph:ralph@localhost:5432/ralph?sslmode=disable}"
+export LISTEN_ADDR="${LISTEN_ADDR:-:8081}"
+
+echo "Starting RALPH backend on $LISTEN_ADDR ..."
+exec go run ./cmd/server