Add reMarkable backup system with web UI

- Docker container with rmapi cloud backup, PDF conversion pipeline
- Web UI: file browser, multi-select, delete/move, thumbnail preview
- Sync: backup from reMarkable cloud, rmdoc→PDF conversion via rmc+Inkscape
- Excluded-files mechanism to prevent deleted items from returning after sync

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

remarkable/entrypoint.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+BASE_DIR="${BASE_DIR:-/data}"
+BACKUP_CRON="${BACKUP_CRON:-15 3 * * *}"
+SNAPSHOT_KEEP="${SNAPSHOT_KEEP:-60}"
+RUN_ON_START="${RUN_ON_START:-0}"
+
+mkdir -p "${BASE_DIR}" /var/log
+
+# Cron-Job einrichten
+CRON_CMD="SNAPSHOT_KEEP=${SNAPSHOT_KEEP} remarkable-backup.sh ${BASE_DIR} / && remarkable-convert.sh ${BASE_DIR}/current >> /var/log/rm-backup.log 2>&1"
+echo "${BACKUP_CRON} root ${CRON_CMD}" > /etc/cron.d/remarkable
+chmod 0644 /etc/cron.d/remarkable
+
+echo "== reMarkable Container =="
+echo "Cron:     ${BACKUP_CRON}"
+echo "Base:     ${BASE_DIR}"
+echo "Retention: ${SNAPSHOT_KEEP}"
+
+if [[ "${RUN_ON_START}" == "1" ]]; then
+  echo "Starte initiales Backup ..."
+  SNAPSHOT_KEEP="${SNAPSHOT_KEEP}" remarkable-backup.sh "${BASE_DIR}" / || true
+  remarkable-convert.sh "${BASE_DIR}/current" || true
+fi
+
+# Cron starten
+service cron start
+
+# PHP Web-Server im Vordergrund
+echo "Web-UI: http://0.0.0.0:8080"
+exec php -S 0.0.0.0:8080 -t /app