feat(ui): Phase D.3 — multi-select + batch actions (pLimit)

- DocumentTable/DocumentCard: selectable/selectedIds/onselectionchange props
  * Table: 왼쪽 6px 너비 체크박스 컬럼
  * Card: 좌상단 absolute 체크박스 (hover 또는 selected 시 표시)
  * 체크박스 onclick stopPropagation으로 행 select와 분리
- documents/+page.svelte:
  * selectedIds = $state(new Set()), URL/필터 변경 시 자동 초기화
  * sticky 선택 toolbar (selection > 0): N건 / 전체 선택 / 선택 해제 /
    일괄 도메인 / 일괄 태그 / 일괄 삭제
  * 50건 상한 UI 가드 (초과 시 경고 + 모든 bulk 버튼 disabled)
  * Bulk modals:
    - 일괄 도메인: Select (Knowledge/* 6종 + Reference)
    - 일괄 태그: TextInput (기존 ai_tags에 추가, 중복 skip)
    - 일괄 삭제: ConfirmDialog (delete_file=true)
  * runBulk 헬퍼: pLimit(5) + Promise.allSettled로 concurrency 제한,
    성공/실패 카운트 toast (5대 원칙 #4)
  * TODO(backend): POST /documents/batch-update — 단일 트랜잭션으로 교체

검증:
- npm run build 통과 (새 경고 없음, label → span 교체로 a11y clean)
- npm run lint:tokens 231 유지 (신규 코드 위반 0)
- 기존 pLimit.ts (Phase A 머지) 재사용, 외부 의존성 없음

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/DocumentTable.svelte

@@ -2,7 +2,23 @@
   import { goto } from '$app/navigation';
   import FormatIcon from './FormatIcon.svelte';
 
-  let { items = [], selectedId = null, onselect = null } = $props();
+  let {
+    items = [],
+    selectedId = null,
+    onselect = null,
+    // D.3 다중 선택
+    selectable = false,
+    selectedIds = new Set(),
+    onselectionchange = null,
+  } = $props();
+
+  function toggleSelection(id, e) {
+    e?.stopPropagation?.();
+    const next = new Set(selectedIds);
+    if (next.has(id)) next.delete(id);
+    else next.add(id);
+    onselectionchange?.(next);
+  }
   let sortKey = $state('created_at');
   let sortOrder = $state('desc');
 
@@ -95,6 +111,9 @@
 <div class="w-full">
   <!-- 헤더 -->
   <div class="flex items-center gap-1 px-2 py-1.5 border-b border-default text-[10px] text-dim uppercase tracking-wider">
+    {#if selectable}
+      <div class="w-6 shrink-0" aria-hidden="true"></div>
+    {/if}
     {#each columns as col}
       <button
         onclick={() => toggleSort(col.key)}
@@ -110,11 +129,29 @@
 
   <!-- 행 -->
   {#each sortedItems() as doc}
-    <button
-      onclick={() => handleClick(doc)}
-      class="flex items-center gap-1 px-2 py-1.5 w-full text-left border-b border-default/30 hover:bg-surface transition-colors group
-        {selectedId === doc.id ? 'bg-accent/5 border-l-2 border-l-accent' : ''}"
+    {@const isChecked = selectedIds.has(doc.id)}
+    <div
+      class="flex items-center gap-1 px-2 py-1.5 w-full border-b border-default/30 hover:bg-surface transition-colors group
+        {selectedId === doc.id ? 'bg-accent/5 border-l-2 border-l-accent' : ''}
+        {isChecked ? 'bg-accent/10' : ''}"
     >
+      {#if selectable}
+        <span class="w-6 shrink-0 flex items-center justify-center">
+          <input
+            type="checkbox"
+            checked={isChecked}
+            onchange={(e) => toggleSelection(doc.id, e)}
+            onclick={(e) => e.stopPropagation()}
+            class="h-3.5 w-3.5 accent-accent cursor-pointer"
+            aria-label="{doc.title || '문서'} 선택"
+          />
+        </span>
+      {/if}
+      <button
+        type="button"
+        onclick={() => handleClick(doc)}
+        class="flex-1 flex items-center gap-1 text-left min-w-0"
+      >
       <!-- 이름 -->
       <div class="flex-1 flex items-center gap-2 min-w-0">
         <span class="w-1 h-4 rounded-full shrink-0" style="background: {getDomainColor(doc.ai_domain)}"></span>
@@ -137,6 +174,7 @@
       <div class="w-20 text-[10px] text-dim text-right">
         {formatDate(doc.created_at)}
       </div>
-    </button>
+      </button>
+    </div>
   {/each}
 </div>