refactor: retire the images tarball backup

Images and attachments are backed up off-site, one object per file, so the
tar+zstd of /images goes: its service and restore, both tasks, the trigger and
restore routes, the Keep last setting, the screen's Images section, and the
tests. Migration 0122 drops backup_run.tar_path and
import_settings.backup_images_keep_last_n and deletes any images runs.

The local database dump stays for an install with no off-site destination,
and the Backups card now says plainly that images and attachments are backed
up only off-site.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/modal/DestructiveConfirmModal.vue

@@ -58,7 +58,7 @@ import { computed, ref, watch } from 'vue'
 const props = defineProps({
   modelValue: { type: Boolean, required: true },
   action: { type: String, required: true },          // 'restore' | 'delete'
-  kind: { type: String, required: true },            // 'db' | 'images' | 'artist' | 'tag' | 'images-selection' | 'audit' | 'min-dim'
+  kind: { type: String, required: true },            // 'db' | 'offsite' | 'artist' | 'tag' | 'images-selection' | 'audit' | 'min-dim'
   runId: { type: [Number, String], default: '' },    // numeric id or sha8 string
   description: { type: String, default: '' },
   tier: { type: String, default: 'C' },              // 'B' | 'C'