fix(comment-cards): 일시적 YouTube 403/5xx 자동 재시도

commentThreads가 order=relevance에서 간헐적으로 403 forbidden을 반환하는 문제 대응.
generic 403/rateLimit/5xx는 백오프 후 최대 2회 재시도, commentsDisabled/quotaExceeded는
재시도 없이 즉시 안내. 재시도 성공/비재시도 단위 테스트 추가.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/main/java/com/hlab/yanalyst/service/YoutubeCommentService.java

@@ -33,6 +33,10 @@ public class YoutubeCommentService {
 
     private static final ObjectMapper MAPPER = new ObjectMapper();
 
+    /** 일시적 오류(transient 403/5xx) 재시도 횟수와 백오프(ms). 테스트에서 0으로 낮춤. */
+    private static final int MAX_RETRIES = 2;
+    private long retryBackoffMs = 600;
+
     public List<CommentCardDto> fetchComments(String urlOrId) {
         String videoId = YoutubeVideoIdParser.parse(urlOrId);
         if (videoId == null) {
@@ -58,7 +62,7 @@ public class YoutubeCommentService {
 
             JsonNode root;
             try {
-                root = restTemplate.getForObject(b.build().encode().toUri(), JsonNode.class);
+                root = fetchPageWithRetry(b.build().encode().toUri());
             } catch (RestClientResponseException e) {
                 // 일부라도 모았으면 그걸로 반환(다음 페이지 실패는 무시)
                 if (!result.isEmpty()) break;
@@ -87,6 +91,43 @@ public class YoutubeCommentService {
         return result;
     }
 
+    /**
+     * 페이지 1개를 호출하되, 일시적 오류(generic 403 forbidden, rateLimitExceeded, 5xx 등)는 재시도한다.
+     * 회복 불가능한 오류(commentsDisabled/quotaExceeded 등)는 즉시 전파해 불필요한 대기를 막는다.
+     */
+    private JsonNode fetchPageWithRetry(java.net.URI uri) {
+        RestClientResponseException last = null;
+        for (int attempt = 0; attempt <= MAX_RETRIES; attempt++) {
+            try {
+                return restTemplate.getForObject(uri, JsonNode.class);
+            } catch (RestClientResponseException e) {
+                if (isNonRetryable(extractReason(e.getResponseBodyAsString()))) {
+                    throw e;
+                }
+                last = e;
+                if (attempt < MAX_RETRIES) {
+                    sleepQuietly(retryBackoffMs * (attempt + 1));
+                }
+            }
+        }
+        throw last;
+    }
+
+    /** 재시도해도 회복되지 않는 reason(영구적 거부). */
+    private static boolean isNonRetryable(String reason) {
+        return "commentsDisabled".equals(reason)
+                || "quotaExceeded".equals(reason)
+                || "dailyLimitExceeded".equals(reason);
+    }
+
+    private static void sleepQuietly(long ms) {
+        try {
+            Thread.sleep(ms);
+        } catch (InterruptedException ie) {
+            Thread.currentThread().interrupt();
+        }
+    }
+
     /** YouTube 오류 응답 본문에서 error.errors[0].reason 추출. 실패 시 null. */
     private static String extractReason(String body) {
         if (body == null || body.isBlank()) return null;

src/test/java/com/hlab/yanalyst/service/YoutubeCommentServiceTest.java

@@ -25,9 +25,19 @@ class YoutubeCommentServiceTest {
         when(guard.tryConsume(anyLong())).thenReturn(true);
         YoutubeCommentService svc = new YoutubeCommentService(rt, guard);
         ReflectionTestUtils.setField(svc, "youtubeApiKey", "TEST_KEY");
+        ReflectionTestUtils.setField(svc, "retryBackoffMs", 0L); // 테스트 속도
         return svc;
     }
 
+    private static final String OK_BODY =
+            "{\"items\":[{\"snippet\":{\"totalReplyCount\":2,\"topLevelComment\":{\"snippet\":{"
+            + "\"authorDisplayName\":\"a\",\"authorProfileImageUrl\":\"https://yt3.ggpht.com/x\","
+            + "\"textDisplay\":\"hi\",\"likeCount\":5,\"publishedAt\":\"2020-01-01T00:00:00Z\"}}}}]}";
+
+    private com.fasterxml.jackson.databind.JsonNode okNode() throws Exception {
+        return new com.fasterxml.jackson.databind.ObjectMapper().readTree(OK_BODY);
+    }
+
     private HttpClientErrorException apiError(HttpStatus status, String reason) {
         String body = "{\"error\":{\"code\":" + status.value()
                 + ",\"message\":\"x\",\"errors\":[{\"reason\":\"" + reason + "\"}]}}";
@@ -56,6 +66,27 @@ class YoutubeCommentServiceTest {
         assertFalse(ex.getMessage().contains("비활성화"), ex.getMessage());
     }
 
+    @Test
+    void transientForbidden_retriesThenSucceeds() throws Exception {
+        RestTemplate rt = mock(RestTemplate.class);
+        when(rt.getForObject(any(URI.class), eq(JsonNode.class)))
+                .thenThrow(apiError(HttpStatus.FORBIDDEN, "forbidden")) // 일시적 403 1회
+                .thenReturn(okNode());                                   // 재시도 성공
+        var out = newService(rt).fetchComments("dQw4w9WgXcQ");
+        assertEquals(1, out.size());
+        assertEquals("hi", out.get(0).getText());
+        verify(rt, times(2)).getForObject(any(URI.class), eq(JsonNode.class));
+    }
+
+    @Test
+    void commentsDisabled_doesNotRetry() {
+        RestTemplate rt = mock(RestTemplate.class);
+        when(rt.getForObject(any(URI.class), eq(JsonNode.class)))
+                .thenThrow(apiError(HttpStatus.FORBIDDEN, "commentsDisabled"));
+        assertThrows(IllegalArgumentException.class, () -> newService(rt).fetchComments("dQw4w9WgXcQ"));
+        verify(rt, times(1)).getForObject(any(URI.class), eq(JsonNode.class)); // 재시도 없음
+    }
+
     @Test
     void invalidKey_throwsMeaningfulMessage_notDisabled() {
         RestTemplate rt = mock(RestTemplate.class);