fix(brain): retry 503 once, don't persist failure message to DB

- analyzeWithGemini: 503（一時高負荷）は4秒後に1回リトライ
- 失敗時は null を返し、呼び出し側で DB を更新しない
  → 「AI分析に失敗しました」がsummaryとして永続化されなくなる

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

server.js

@@ -428,8 +428,8 @@ function smartExtract(text, maxLen) {
     return front + '\n\n[...中略...]\n\n' + back;
 }
 
-async function analyzeWithGemini(title, fullText, url) {
-    if (!genAI) return { summary: (fullText || '').slice(0, 120) || '（要約なし）', topics: ['その他'], readingTime: 3 };
+async function analyzeWithGemini(title, fullText, url, _retry = false) {
+    if (!genAI) return null;
     try {
         const model = genAI.getGenerativeModel({
             model: 'gemini-2.5-flash',
@@ -471,11 +471,13 @@ ${smartExtract(fullText || '', 5000)}
         if (typeof result !== 'undefined' && result?.response) {
             console.error('[Gemini] Raw response:', result.response.text());
         }
-        return {
-            summary: 'AI分析に失敗しました。しばらく後にお試しください。',
-            topics: ['その他'],
-            readingTime: 3
-        };
+        // 503（一時的高負荷）は1回だけリトライ
+        if (!_retry && e.status === 503) {
+            console.warn('[Gemini] 503 detected, retrying in 4s...');
+            await new Promise(r => setTimeout(r, 4000));
+            return analyzeWithGemini(title, fullText, url, true);
+        }
+        return null;
     }
 }
 
@@ -1463,6 +1465,7 @@ function buildRouter() {
 
                     if (checkRateLimit('gemini_analyze', savedUserId, 50, 60 * 60 * 1000)) {
                         analyzeWithGemini(finalTitle, fullText || meta.desc, url).then(async (ai) => {
+                            if (!ai) { console.warn(`[Brain API] AI analysis skipped (null) for ${url}`); return; }
                             await pool.query(`
                                 UPDATE articles SET summary=$1, topics=$2, reading_time=$3
                                 WHERE user_id=$4 AND url=$5
@@ -1563,6 +1566,7 @@ function buildRouter() {
 
                     if (checkRateLimit('gemini_analyze', savedUserId, 50, 60 * 60 * 1000)) {
                         analyzeWithGemini(meta.title, fullText, url).then(async (ai) => {
+                            if (!ai) { console.warn(`[Brain API] AI analysis skipped (null) for ${url}`); return; }
                             await pool.query(`
                                 UPDATE articles SET summary=$1, topics=$2, reading_time=$3
                                 WHERE user_id=$4 AND url=$5