main主干首次提交，包含用户认证模块

src/main/java/com/labelsys/backend/util/IdGenerator.java

@@ -0,0 +1,16 @@
+package com.labelsys.backend.util;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+public final class IdGenerator {
+
+    private static final AtomicInteger SEQUENCE = new AtomicInteger();
+
+    private IdGenerator() {
+    }
+
+    public static long nextId() {
+        int sequence = SEQUENCE.updateAndGet(value -> value >= 999 ? 0 : value + 1);
+        return System.currentTimeMillis() * 1000 + sequence;
+    }
+}