refactor(admin): split admin page into tab sub-components

Split admin/+page.svelte (573 lines) into:
- UsersTab.svelte: user table with delete action
- TagsTab.svelte: tag list with inline rename and delete
- GroupsTab.svelte: groups table with inline edit + create form
- SystemTab.svelte: backfill buttons with own state

Page drops from 573 → ~40 lines.

Part of #75

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

frontend/src/routes/admin/SystemTab.svelte

@@ -0,0 +1,72 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+
+let backfillResult: number | null = $state(null);
+let backfillLoading = $state(false);
+let backfillHashesResult: number | null = $state(null);
+let backfillHashesLoading = $state(false);
+
+async function backfillVersions() {
+	backfillLoading = true;
+	backfillResult = null;
+	try {
+		const res = await fetch('/api/admin/backfill-versions', { method: 'POST' });
+		if (res.ok) {
+			const data = await res.json();
+			backfillResult = data.count;
+		}
+	} finally {
+		backfillLoading = false;
+	}
+}
+
+async function backfillFileHashes() {
+	backfillHashesLoading = true;
+	backfillHashesResult = null;
+	try {
+		const res = await fetch('/api/admin/backfill-file-hashes', { method: 'POST' });
+		if (res.ok) {
+			const data = await res.json();
+			backfillHashesResult = data.count;
+		}
+	} finally {
+		backfillHashesLoading = false;
+	}
+}
+</script>
+
+<div class="rounded-sm border border-line bg-surface p-6 shadow-sm">
+	<h2 class="mb-1 text-lg font-bold text-ink-2">{m.admin_system_backfill_heading()}</h2>
+	<p class="mb-4 text-sm text-ink-2">{m.admin_system_backfill_description()}</p>
+	<button
+		onclick={backfillVersions}
+		disabled={backfillLoading}
+		class="rounded bg-primary px-6 py-2 text-sm font-bold text-white uppercase transition hover:bg-accent hover:text-ink disabled:cursor-not-allowed disabled:opacity-50"
+	>
+		{backfillLoading ? '…' : m.admin_system_backfill_btn()}
+	</button>
+	{#if backfillResult !== null}
+		<p class="mt-4 text-sm font-medium text-ink">
+			{m.admin_system_backfill_success({ count: backfillResult })}
+		</p>
+	{/if}
+</div>
+
+<div class="mt-4 rounded-sm border border-line bg-surface p-6 shadow-sm">
+	<h2 class="mb-1 text-lg font-bold text-ink-2">
+		{m.admin_system_backfill_hashes_heading()}
+	</h2>
+	<p class="mb-4 text-sm text-ink-2">{m.admin_system_backfill_hashes_description()}</p>
+	<button
+		onclick={backfillFileHashes}
+		disabled={backfillHashesLoading}
+		class="rounded bg-primary px-6 py-2 text-sm font-bold text-white uppercase transition hover:bg-accent hover:text-ink disabled:cursor-not-allowed disabled:opacity-50"
+	>
+		{backfillHashesLoading ? '…' : m.admin_system_backfill_hashes_btn()}
+	</button>
+	{#if backfillHashesResult !== null}
+		<p class="mt-4 text-sm font-medium text-ink">
+			{m.admin_system_backfill_hashes_success({ count: backfillHashesResult })}
+		</p>
+	{/if}
+</div>