fix(render): honor WEB_CONCURRENCY for worker count

Render auto-sets WEB_CONCURRENCY based on the instance's CPU/memory (1 on
the free tier). Hardcoding --workers 4 caused workers to be OOM-killed
on the free tier — the parent kept respawning replacements, producing a
steady stream of "Child process died" messages and wasted resources.
Default to 1 if WEB_CONCURRENCY is unset.

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

Author: Ho1yShif

backend/scripts/start.sh

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