Add backup shrinking functionality to eMMC provisioning dashboard: implement API for shrinking raw .img backups using PiShrink, update UI to support shrink option after backup, and enhance documentation for backup image handling and storage options on Proxmox host.

chromium-setup/emmc-provisioning/scripts/install-pishrink-on-host.sh

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+#!/usr/bin/env bash
+# Run on the provisioning HOST (root) to install PiShrink and dependencies.
+# Enables shrinking backups in flash-emmc-on-connect.sh when SHRINK_BACKUP=1.
+# PiShrink: https://github.com/Drewsif/PiShrink
+
+set -e
+PISHRINK_URL="${PISHRINK_URL:-https://raw.githubusercontent.com/Drewsif/PiShrink/master/pishrink.sh}"
+
+echo "Installing PiShrink dependencies..."
+apt-get update
+apt-get install -y wget parted gzip pigz xz-utils udev e2fsprogs
+
+echo "Downloading PiShrink..."
+wget -q -O /usr/local/bin/pishrink.sh "$PISHRINK_URL"
+chmod +x /usr/local/bin/pishrink.sh
+
+echo "PiShrink installed at /usr/local/bin/pishrink.sh"
+echo "To shrink backups automatically, add to /opt/cm4-provisioning/env:"
+echo "  SHRINK_BACKUP=1"
+echo "  # optional: PISHRINK_COMPRESS=gz   or   PISHRINK_COMPRESS=xz"