feat(#1008): archive auto-purge + dedicated archives:purge permission

Adds an archive counterpart to the library trash sweeper shipped in the
  previous commit. Unlike the library flow, archives are hard-deleted —
  print history is a decaying timeline, so there is no trash intermediate;
  download or favourite anything you want to keep first.

  Backend
  - New ArchivePurgeService (backend/app/services/archive_purge.py) with
    its own 15-minute scheduler loop and a 24h throttle on actual purge
    runs. Delegates every delete to the existing safety-checked
    ArchiveService.delete_archive so the 3MF, thumbnail, timelapse, source
    3MF, F3D, and photo folder all get cleaned up together with the DB
    row. Per-row session via async_session() avoids commit-per-row churn
    on any caller-passed session.
  - New /archives/purge/{preview,settings} + POST /archives/purge routes
    gated on a dedicated archives:purge permission (not archives:delete_all)
    so admins can delegate bulk-delete to a role without granting
    per-archive delete on other users' rows.
  - seed_default_groups() now backfills both library:purge and
    archives:purge on the Administrators group for upgraded installs —
    the original library:purge was added after Administrators was first
    seeded so the "create if not exists" path skipped existing DBs and
    left admins without the permission.
  - 8 new integration tests (defaults, settings roundtrip, bound
    validation, preview, manual purge, auto-purge enabled path, 24h
    throttle, disabled skip).

  Frontend
  - Settings → Archives card gains an auto-purge toggle + age input (7d
    floor, 10y ceiling, 365d default), with a save-toast on every change.
    The bulk "Purge old" button lives on the Archives page header
    (rightmost, after Upload 3MF) to match the File Manager pattern —
    configuration in Settings, one-shot action on the page.
  - New PurgeArchivesModal mirrors PurgeOldFilesModal: live preview (count
    + total size freed + sample filenames) debounced at 300ms, amber
    "hard-delete, no undo" warning.
  - Admin-only UI gates on archives:purge via the standard hasPermission
    hook; Permission TS union updated.
  - i18n blocks across all 8 locales (en/de full, other 6 English
    fallback per project convention).

  Docs
  - CHANGELOG entry under 0.2.4b1 following the existing library-trash
    entry.
  - bambuddy-wiki archiving.md gains a new "Auto-Purge" section.
  - bambuddy-website features.html gets a matching bullet.

  Verification: python -m ruff check backend/app/ clean; 25 integration
  tests pass (8 archive_purge + 17 library_trash regression); npm run
  build clean.

frontend/src/components/PurgeArchivesModal.tsx

@@ -0,0 +1,160 @@
+import { useEffect, useState } from 'react';
+import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
+import { useTranslation } from 'react-i18next';
+import { AlertTriangle, Loader2, Trash2, X } from 'lucide-react';
+
+import { api } from '../api/client';
+import { Button } from './Button';
+import { useToast } from '../contexts/ToastContext';
+import { formatFileSize } from '../utils/file';
+
+interface PurgeArchivesModalProps {
+  onClose: () => void;
+  initialDays?: number;
+}
+
+const DEFAULT_DAYS = 365;
+
+export function PurgeArchivesModal({ onClose, initialDays }: PurgeArchivesModalProps) {
+  const { t } = useTranslation();
+  const queryClient = useQueryClient();
+  const { showToast } = useToast();
+
+  const [days, setDays] = useState(initialDays ?? DEFAULT_DAYS);
+
+  const [debouncedDays, setDebouncedDays] = useState(days);
+  useEffect(() => {
+    const handle = window.setTimeout(() => setDebouncedDays(days), 300);
+    return () => window.clearTimeout(handle);
+  }, [days]);
+
+  const previewQuery = useQuery({
+    queryKey: ['archive-purge-preview', debouncedDays],
+    queryFn: () => api.previewArchivePurge(debouncedDays),
+    enabled: debouncedDays >= 1,
+  });
+
+  const purgeMutation = useMutation({
+    mutationFn: () => api.executeArchivePurge(days),
+    onSuccess: (res) => {
+      showToast(t('archivePurge.toast.success', { count: res.deleted }), 'success');
+      queryClient.invalidateQueries({ queryKey: ['archives'] });
+      queryClient.invalidateQueries({ queryKey: ['archive-stats'] });
+      onClose();
+    },
+    onError: (e: Error) => showToast(e.message || t('archivePurge.toast.failed'), 'error'),
+  });
+
+  useEffect(() => {
+    const handleKey = (e: KeyboardEvent) => {
+      if (e.key === 'Escape' && !purgeMutation.isPending) onClose();
+    };
+    window.addEventListener('keydown', handleKey);
+    return () => window.removeEventListener('keydown', handleKey);
+  }, [onClose, purgeMutation.isPending]);
+
+  const preview = previewQuery.data;
+  const count = preview?.count ?? 0;
+  const totalBytes = preview?.total_bytes ?? 0;
+  const canConfirm = count > 0 && !purgeMutation.isPending;
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/50 p-4">
+      <div className="bg-white dark:bg-gray-900 rounded-lg shadow-xl max-w-lg w-full">
+        <div className="flex items-center justify-between p-4 border-b border-gray-200 dark:border-gray-700">
+          <h2 className="text-lg font-semibold text-gray-900 dark:text-gray-100 flex items-center gap-2">
+            <Trash2 className="w-5 h-5" />
+            {t('archivePurge.title')}
+          </h2>
+          <button
+            onClick={onClose}
+            className="text-gray-400 hover:text-gray-600 dark:hover:text-gray-200"
+            aria-label={t('common.close')}
+            disabled={purgeMutation.isPending}
+          >
+            <X className="w-5 h-5" />
+          </button>
+        </div>
+
+        <div className="p-4 space-y-4">
+          <p className="text-sm text-gray-600 dark:text-gray-400">
+            {t('archivePurge.description')}
+          </p>
+
+          <div>
+            <label htmlFor="archive-purge-days" className="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-1">
+              {t('archivePurge.ageLabel')}
+            </label>
+            <div className="flex items-center gap-3">
+              <input
+                id="archive-purge-days"
+                type="number"
+                min={1}
+                max={3650}
+                value={days}
+                onChange={(e) => setDays(Math.max(1, Math.min(3650, parseInt(e.target.value || '0', 10) || 0)))}
+                className="w-24 rounded border border-gray-300 dark:border-gray-600 bg-white dark:bg-gray-800 text-sm px-2 py-1 text-gray-900 dark:text-gray-100"
+              />
+              <span className="text-sm text-gray-600 dark:text-gray-400">{t('archivePurge.days')}</span>
+            </div>
+          </div>
+
+          <div className="rounded border border-gray-200 dark:border-gray-700 bg-gray-50 dark:bg-gray-800/50 p-3">
+            {previewQuery.isLoading || previewQuery.isFetching ? (
+              <div className="flex items-center gap-2 text-sm text-gray-500 dark:text-gray-400">
+                <Loader2 className="w-4 h-4 animate-spin" /> {t('archivePurge.previewLoading')}
+              </div>
+            ) : previewQuery.isError ? (
+              <div className="text-sm text-red-600 dark:text-red-400">
+                {(previewQuery.error as Error | null)?.message ?? t('archivePurge.previewFailed')}
+              </div>
+            ) : (
+              <div className="text-sm text-gray-900 dark:text-gray-100">
+                <div className="font-medium">
+                  {t('archivePurge.previewSummary', { count, size: formatFileSize(totalBytes) })}
+                </div>
+                {preview?.sample_filenames && preview.sample_filenames.length > 0 && (
+                  <ul className="mt-2 text-xs text-gray-600 dark:text-gray-400 space-y-0.5 list-disc pl-4">
+                    {preview.sample_filenames.map((name) => (
+                      <li key={name} className="truncate">{name}</li>
+                    ))}
+                    {count > preview.sample_filenames.length && (
+                      <li className="list-none italic text-gray-500">
+                        {t('archivePurge.andMore', { count: count - preview.sample_filenames.length })}
+                      </li>
+                    )}
+                  </ul>
+                )}
+              </div>
+            )}
+          </div>
+
+          <div className="flex gap-2 items-start text-xs text-amber-700 dark:text-amber-400 bg-amber-50 dark:bg-amber-900/20 rounded px-3 py-2">
+            <AlertTriangle className="w-4 h-4 mt-0.5 shrink-0" />
+            <span>{t('archivePurge.warning')}</span>
+          </div>
+        </div>
+
+        <div className="flex justify-end gap-2 p-4 border-t border-gray-200 dark:border-gray-700">
+          <Button variant="secondary" onClick={onClose} disabled={purgeMutation.isPending}>
+            {t('common.cancel')}
+          </Button>
+          <Button
+            variant="danger"
+            disabled={!canConfirm}
+            onClick={() => purgeMutation.mutate()}
+          >
+            {purgeMutation.isPending ? (
+              <>
+                <Loader2 className="w-4 h-4 animate-spin mr-1" />
+                {t('archivePurge.purging')}
+              </>
+            ) : (
+              t('archivePurge.confirmCta', { count })
+            )}
+          </Button>
+        </div>
+      </div>
+    </div>
+  );
+}