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>

.gitignore

@@ -85,3 +85,5 @@ clients/voice-confirmation/dist/
 logs/
 Ultralytics/
 scripts/
+!backend/scripts/
+!backend/scripts/rebuild-api-image.sh