feat(t15): 健康检查端点 /health（pg/redis/队列四态，DOWN 返503）

- DistQueueMapper: +countByStatus (GROUP BY status 一次拿全)
- HealthService(新): 聚合 pg 连通/redis ping/队列四态，status UP/DEGRADED/DOWN
- HealthController(新): GET /health，DOWN 返 503 便 LB 摘流；免租户鉴权(PathWhitelist)
- 联调：status=UP，pg/redis=UP，distQueue 四态 0，HTTP 200

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

src/main/java/com/imutil/mapper/DistQueueMapper.java

@@ -78,4 +78,12 @@ public interface DistQueueMapper extends BaseMapper<DistQueue> {
             "locked_by = NULL, locked_at = NULL, updated_at = now() " +
             "WHERE id = #{id} AND status = 3")
     int redeliverDead(@Param("id") Long id);
+
+    /**
+     * 按状态聚合计数（健康检查/监控用，一次扫描拿全状态分布）
+     *
+     * @return 每项含 status（0=pending/1=processing/2=done/3=dead）与 cnt
+     */
+    @org.apache.ibatis.annotations.Select("SELECT status, count(*) AS cnt FROM dist_queue GROUP BY status")
+    java.util.List<java.util.Map<String, Object>> countByStatus();
 }