fix(render): restore start.sh removed upstream

Upstream deleted backend/scripts/start.sh and split its responsibilities:
the Dockerfile CMD now runs fastapi directly, and prestart.sh runs in a
separate compose service. Render has no equivalent of "wait for sidecar to
complete," and the existing Render service is configured with
`bash scripts/start.sh` as its Docker command. Restore the file with its
prior contents so the deploy works without any Dashboard config changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Ho1yShif

backend/scripts/start.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+set -e
+bash scripts/prestart.sh
+exec fastapi run --workers 4 --host 0.0.0.0 --port "$PORT" app/main.py