feat(bulk-edit): add /documents/bulk-edit route

Server load redirects READ_ALL-only users (or unauthenticated) to /documents.
Page load: onMount reads bulkSelectionStore — redirects to /documents when the
store is empty, otherwise POSTs the IDs to /api/documents/batch-metadata and
hands the resulting summaries to BulkDocumentEditLayout in mode="edit".

Refs #225

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

frontend/src/routes/documents/bulk-edit/+page.svelte

@@ -0,0 +1,53 @@
+<script lang="ts">
+import { onMount } from 'svelte';
+import { goto } from '$app/navigation';
+import { bulkSelectionStore } from '$lib/stores/bulkSelection.svelte';
+import BulkDocumentEditLayout, {
+	type BulkEditEntry
+} from '$lib/components/document/BulkDocumentEditLayout.svelte';
+import { m } from '$lib/paraglide/messages.js';
+
+let entries = $state<BulkEditEntry[]>([]);
+let loading = $state(true);
+let error = $state<string | null>(null);
+
+onMount(async () => {
+	const ids = Array.from(bulkSelectionStore.ids);
+	if (ids.length === 0) {
+		await goto('/documents');
+		return;
+	}
+	try {
+		const res = await fetch('/api/documents/batch-metadata', {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/json' },
+			body: JSON.stringify({ ids })
+		});
+		if (!res.ok) {
+			error = m.error_internal_error();
+			loading = false;
+			return;
+		}
+		const summaries = (await res.json()) as BulkEditEntry[];
+		entries = summaries;
+	} catch {
+		error = m.error_internal_error();
+	} finally {
+		loading = false;
+	}
+});
+</script>
+
+<svelte:head>
+	<title>{m.bulk_edit_title()} – Familienarchiv</title>
+</svelte:head>
+
+{#if loading}
+	<div class="flex h-full items-center justify-center p-12 text-sm text-ink-2">…</div>
+{:else if error}
+	<div class="m-6 rounded-sm border border-red-300 bg-red-50 p-4 text-sm text-red-700">
+		{error}
+	</div>
+{:else if entries.length > 0}
+	<BulkDocumentEditLayout mode="edit" initialEditEntries={entries} />
+{/if}