feat(frontend): add System tab to admin panel with backfill-versions action

Admin can trigger an initial history snapshot for all documents without
version history. Shows count of backfilled documents after completion.

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

frontend/src/lib/generated/api.ts

@@ -228,6 +228,22 @@ export interface paths {
         patch?: never;
         trace?: never;
     };
+    "/api/admin/backfill-versions": {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        get?: never;
+        put?: never;
+        post: operations["backfillVersions"];
+        delete?: never;
+        options?: never;
+        head?: never;
+        patch?: never;
+        trace?: never;
+    };
     "/api/groups/{id}": {
         parameters: {
             query?: never;
@@ -548,6 +564,10 @@ export interface components {
             /** Format: date-time */
             startedAt?: string;
         };
+        BackfillResult: {
+            /** Format: int32 */
+            count: number;
+        };
         DocumentVersionSummary: {
             /** Format: uuid */
             id: string;
@@ -1106,6 +1126,26 @@ export interface operations {
             };
         };
     };
+    backfillVersions: {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        requestBody?: never;
+        responses: {
+            /** @description OK */
+            200: {
+                headers: {
+                    [name: string]: unknown;
+                };
+                content: {
+                    "*/*": components["schemas"]["BackfillResult"];
+                };
+            };
+        };
+    };
     deleteGroup: {
         parameters: {
             query?: never;

frontend/src/routes/admin/+page.svelte

@@ -9,6 +9,8 @@ let activeTab = $state('users');
 let editingTagId: string | null = $state(null);
 let editingTagName = $state('');
 let editingGroupId: string | null = $state(null);
+let backfillResult: number | null = $state(null);
+let backfillLoading = $state(false);
 
 const availablePermissions = ['WRITE_ALL', 'ADMIN', 'ADMIN_USER', 'ADMIN_TAG', 'ADMIN_PERMISSION'];
 
@@ -29,6 +31,20 @@ function startEditGroup(id: string) {
 function cancelEditGroup() {
 	editingGroupId = null;
 }
+
+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;
+	}
+}
 </script>
 
 <div class="mx-auto max-w-7xl py-8 font-sans sm:px-6 lg:px-8">
@@ -58,6 +74,13 @@ function cancelEditGroup() {
 					: 'text-gray-500 hover:text-brand-navy'}"
 				onclick={() => (activeTab = 'tags')}>{m.admin_tab_tags()}</button
 			>
+			<button
+				class="rounded-md px-4 py-2 text-sm font-bold tracking-wide uppercase transition {activeTab ===
+				'system'
+					? 'bg-brand-navy text-white'
+					: 'text-gray-500 hover:text-brand-navy'}"
+				onclick={() => (activeTab = 'system')}>{m.admin_tab_system()}</button
+			>
 		</div>
 	</div>
 
@@ -495,5 +518,22 @@ function cancelEditGroup() {
 				</form>
 			</div>
 		</div>
+	{:else if activeTab === 'system'}
+		<div class="rounded-sm border border-brand-sand bg-white p-6 shadow-sm">
+			<h2 class="mb-1 text-lg font-bold text-gray-700">{m.admin_system_backfill_heading()}</h2>
+			<p class="mb-4 text-sm text-gray-500">{m.admin_system_backfill_description()}</p>
+			<button
+				onclick={backfillVersions}
+				disabled={backfillLoading}
+				class="rounded bg-brand-navy px-6 py-2 text-sm font-bold text-white uppercase transition hover:bg-brand-mint hover:text-brand-navy 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-brand-navy">
+					{m.admin_system_backfill_success({ count: backfillResult })}
+				</p>
+			{/if}
+		</div>
 	{/if}
 </div>