Add S3 CDN media readiness and cache health checks

.env.example

@@ -60,6 +60,9 @@ S3_ENDPOINT=
 S3_ACCESS_KEY_ID=
 S3_SECRET_ACCESS_KEY=
 S3_FORCE_PATH_STYLE=false
+# Optional: writes then deletes a tiny health object when GET /media/health runs.
+# Keep false unless you explicitly want to verify S3 write permissions.
+S3_HEALTH_WRITE_TEST_ENABLED=false
 IMAGE_PROCESSING_ENABLED=true
 IMAGE_PROCESSING_FFMPEG_PATH=ffmpeg
 IMAGE_PROCESSING_LOW_WIDTH=360