fix(backend): AIService 舌诊 JSON 序列化捕获受检异常

预置编译错误（来自 47f471f5b 舌诊 AI 集成），阻塞统一指标层计划全部 11 个任务的
mvn clean compile 验证门。最小修复：writeValueAsString 包 try/catch，失败返回 null。

cfc-backend/src/main/java/com/etotem/cfc/service/AIService.java

@@ -327,7 +327,14 @@ public class AIService {
             headers.setBearerAuth(aikansheToken);
         }
 
-        HttpEntity<String> entity = new HttpEntity<>(objectMapper.writeValueAsString(body), headers);
+        String bodyJson;
+        try {
+            bodyJson = objectMapper.writeValueAsString(body);
+        } catch (Exception e) {
+            log.warn("愛看舌 API 请求体序列化失败: {}", e.getMessage());
+            return null;
+        }
+        HttpEntity<String> entity = new HttpEntity<>(bodyJson, headers);
 try {
             ResponseEntity<String> response = restTemplate.postForEntity(url, entity, String.class);
             if (response.getStatusCode().is2xxSuccessful() && response.getBody() != null) {