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/docker-compose.yml

@@ -0,0 +1,22 @@
+services:
+  remarkable:
+    build: .
+    container_name: remarkable
+    volumes:
+      - notizen:/data
+      - rmapi-config:/root/.config/rmapi
+    environment:
+      - BASE_DIR=/data
+      - BACKUP_CRON=15 3 * * *
+      - SNAPSHOT_KEEP=60
+      - RUN_ON_START=0
+      - RM_BACKUP_ROOT=/data/current
+      - RM_USER=${RM_USER}
+      - RM_PASS=${RM_PASS}
+    ports:
+      - "8980:8080"
+    restart: unless-stopped
+
+volumes:
+  notizen:
+  rmapi-config: