M4: admin settings page and JSON API

- AdminApiController (admin only, CSRF-protected): status, create key,
  download recovery kit, confirm (statement accepted + exact phrase),
  backends with their usable auth mechanisms, add/test/remove locations,
  start backups, schedule, snapshots, browse, queue restores
- RestoreJob runs web-initiated restores in the background; backup and
  restore runs are kept apart in the run table
- admin section 'NG Backup' with a Vue 3 / @nextcloud/vue 9 page: key and
  recovery-kit flow (download, tick statement, 'Yes, I confirm'), location
  form built from files_external's parameter definitions, schedule, run
  list with polling, snapshot browser and restore with the three modes
- auth mechanisms that depend on a user session or on files_external's
  own stored credentials are not offered

API tested with curl on NC34 (status, backends, schedule validation,
confirmation refusal, browse, queued restore and backup completed by
cron); non-admins get 403, anonymous 401.

src/AdminSettings.vue

@@ -0,0 +1,62 @@
+<!--
+  - SPDX-FileCopyrightText: 2026 Dillard Blom
+  - SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+<template>
+	<div class="ng-backup">
+		<NcSettingsSection :name="t('ng_backup', 'NG Backup')"
+			:description="t('ng_backup', 'Encrypted, incremental backups of this Nextcloud (database, configuration and data). Backup locations are never mounted and are not visible in Files.')">
+			<NcLoadingIcon v-if="!status" :size="32" />
+			<NcNoteCard v-if="error" type="error">{{ error }}</NcNoteCard>
+		</NcSettingsSection>
+
+		<template v-if="status">
+			<KeySetup :status="status" @changed="refresh" />
+			<template v-if="status.key.confirmation">
+				<Locations :status="status" @changed="refresh" />
+				<Backups v-if="status.targets.length" :status="status" @changed="refresh" />
+				<RestoreBrowser v-if="status.targets.length" :status="status" @changed="refresh" />
+			</template>
+		</template>
+	</div>
+</template>
+
+<script setup>
+import { onBeforeUnmount, onMounted, ref } from 'vue'
+import { t } from '@nextcloud/l10n'
+import NcLoadingIcon from '@nextcloud/vue/components/NcLoadingIcon'
+import NcNoteCard from '@nextcloud/vue/components/NcNoteCard'
+import NcSettingsSection from '@nextcloud/vue/components/NcSettingsSection'
+import api from './api.js'
+import Backups from './components/Backups.vue'
+import KeySetup from './components/KeySetup.vue'
+import Locations from './components/Locations.vue'
+import RestoreBrowser from './components/RestoreBrowser.vue'
+
+const status = ref(null)
+const error = ref('')
+let timer = null
+
+async function refresh() {
+	try {
+		status.value = await api.status()
+		error.value = ''
+	} catch (e) {
+		error.value = e.message
+	}
+	// Poll while something runs, so progress shows up without reloading.
+	clearTimeout(timer)
+	if (status.value?.runs.some(r => r.status === 'running')) {
+		timer = setTimeout(refresh, 4000)
+	}
+}
+
+onMounted(refresh)
+onBeforeUnmount(() => clearTimeout(timer))
+</script>
+
+<style scoped>
+.ng-backup :deep(.settings-section) {
+	max-width: 900px;
+}
+</style>