feat: 뉴스 필터 트리 (신문사 → 분야) + ai_summary 우선 표시

- 좌측 필터: 신문사 펼침 → 분야별 필터 (News/경향신문/문화)
- API: source 파라미터 '신문사' 또는 '신문사/분야' 지원
- 리스트: ai_summary 있으면 우선, 없으면 extracted_text fallback

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

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

@@ -20,18 +20,23 @@
   let selectedArticle = $state(null);
   let filterSource = $state('');
   let showUnreadOnly = $state(false);
-  let sources = $state([]);
+  let sourceTree = $state({});  // { 경향신문: ['문화', '사회'], NYT: ['World'] }
   let currentPage = $state(1);
   let noteEditing = $state(false);
   let noteText = $state('');
+  let expandedSources = $state({});
 
   onMount(async () => {
     try {
       const srcData = await api('/news/sources');
-      // 신문사별 유니크
-      const names = new Set();
-      srcData.forEach(s => names.add(s.name.split(' ')[0]));
-      sources = [...names];
+      const tree = {};
+      srcData.forEach(s => {
+        const paper = s.name.split(' ')[0];
+        const cat = s.category || '';
+        if (!tree[paper]) tree[paper] = [];
+        if (cat && !tree[paper].includes(cat)) tree[paper].push(cat);
+      });
+      sourceTree = tree;
     } catch (e) {}
     loadArticles();
   });
@@ -127,11 +132,26 @@
       class="w-full text-left px-2 py-1.5 rounded text-sm mb-1 {filterSource === '' ? 'bg-[var(--accent)]/15 text-[var(--accent)]' : 'text-[var(--text-dim)] hover:bg-[var(--surface)]'}"
     >📰 전체</button>
 
-    {#each sources as src}
-      <button
-        onclick={() => { filterSource = src; }}
-        class="w-full text-left px-2 py-1.5 rounded text-sm mb-0.5 {filterSource === src ? 'bg-[var(--accent)]/15 text-[var(--accent)]' : 'text-[var(--text-dim)] hover:bg-[var(--surface)]'}"
-      >{src}</button>
+    {#each Object.entries(sourceTree) as [paper, categories]}
+      <div class="mb-0.5">
+        <button
+          onclick={() => { filterSource = paper; expandedSources[paper] = !expandedSources[paper]; }}
+          class="w-full text-left px-2 py-1.5 rounded text-sm flex items-center justify-between
+            {filterSource === paper ? 'bg-[var(--accent)]/15 text-[var(--accent)]' : 'text-[var(--text-dim)] hover:bg-[var(--surface)]'}"
+        >
+          <span>{paper}</span>
+          <span class="text-[10px]">{expandedSources[paper] ? '▼' : '▶'}</span>
+        </button>
+        {#if expandedSources[paper] && categories.length > 0}
+          {#each categories as cat}
+            <button
+              onclick={() => { filterSource = `${paper}/${cat}`; }}
+              class="w-full text-left pl-6 pr-2 py-1 rounded text-xs
+                {filterSource === `${paper}/${cat}` ? 'bg-[var(--accent)]/15 text-[var(--accent)]' : 'text-[var(--text-dim)] hover:bg-[var(--surface)]'}"
+            >{cat}</button>
+          {/each}
+        {/if}
+      </div>
     {/each}
 
     <hr class="my-3 border-[var(--border)]">