feat(v1): unify API response, validation, exception handling and tests

src/main/java/com/example/demo/common/ApiResponse.java

@@ -0,0 +1,22 @@
+package com.example.demo.common;
+
+import java.time.Instant;
+
+public record ApiResponse<T>(
+        int code,
+        String message,
+        T data,
+        Instant timestamp
+) {
+    public static <T> ApiResponse<T> ok(T data) {
+        return new ApiResponse<>(0, "success", data, Instant.now());
+    }
+
+    public static <T> ApiResponse<T> ok(String message, T data) {
+        return new ApiResponse<>(0, message, data, Instant.now());
+    }
+
+    public static ApiResponse<Void> fail(int code, String message) {
+        return new ApiResponse<>(code, message, null, Instant.now());
+    }
+}