功能追加：显示在线人数

backend/src/main/java/com/openisle/controller/OnlineController.java

@@ -0,0 +1,33 @@
+package com.openisle.controller;
+
+import com.openisle.config.CachingConfig;
+import lombok.RequiredArgsConstructor;
+import org.springframework.data.redis.core.RedisTemplate;
+import org.springframework.data.redis.core.StringRedisTemplate;
+import org.springframework.web.bind.annotation.*;
+
+import java.time.Duration;
+
+/**
+ * @author smallclover
+ * @since 2025-09-05
+ * 统计在线人数
+ */
+@RestController
+@RequestMapping("/api/online")
+@RequiredArgsConstructor
+public class OnlineController {
+
+    private final RedisTemplate redisTemplate;
+    private static final String ONLINE_KEY = CachingConfig.ONLINE_CACHE_NAME +":";
+
+    @PostMapping("/heartbeat")
+    public void ping(@RequestParam String userId){
+        redisTemplate.opsForValue().set(ONLINE_KEY+userId,"1", Duration.ofSeconds(150));
+    }
+
+    @GetMapping("/count")
+    public long count(){
+        return redisTemplate.keys(ONLINE_KEY+"*").size();
+    }
+}