Align API container UID with host and harden RTSP slice readiness.

Run compose api as HOST_UID/GID with cache under /tmp, poll slice files for
ready_event when ffmpeg stderr is silent, invoke batch via venv python, exclude
logs from build context, and document Docker cache/VLC troubleshooting.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/scripts/rebuild-api-image.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Clear BuildKit cache and rebuild the API image (fixes corrupted layer / unpigz errors).
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+echo "Pruning BuildKit cache..."
+docker builder prune -af
+docker buildx prune -af 2>/dev/null || true
+docker rmi -f backend-api:latest 2>/dev/null || true
+
+if [[ "${RESTART_DOCKER:-0}" == "1" ]]; then
+  echo "Restarting Docker..."
+  sudo systemctl restart docker
+fi
+
+echo "Building api image (--no-cache)..."
+docker compose build api --no-cache
+
+echo "Done. Recreate container: docker compose up -d --force-recreate api"