fix(admin): Correct admin initialization and password salt compatibility

backend/video-admin/src/main/java/com/nuaa/video/admin/service/impl/AdminUserInitService.java

@@ -49,7 +49,7 @@ public class AdminUserInitService implements ApplicationRunner {
         adminUser.setRealName("系统管理员");
         adminUser.setEmail("admin@example.com");
         adminUser.setStatus(1); // 正常状态
-        adminUser.setSalt(null);
+        adminUser.setSalt(passwordService.generateSalt());
         adminUser.setCreatedBy("system");
         adminUser.setUpdatedBy("system");
         adminUser.setCreateTime(LocalDateTime.now());

backend/video-admin/src/main/java/com/nuaa/video/admin/service/impl/AdminUserServiceImpl.java

@@ -221,7 +221,7 @@ public class AdminUserServiceImpl extends ServiceImpl<AdminUserMapper, AdminUser
         } else {
             // 不更新密码
             adminUser.setPassword(null);
-            adminUser.setSalt(null); // 同时不更新盐值
+            adminUser.setSalt(""); // 使用空串兼容非空字段约束
         }
 
         adminUser.setUpdatedBy(operator);

backend/video-admin/src/main/java/com/nuaa/video/admin/service/impl/PasswordServiceImpl.java

@@ -20,7 +20,7 @@ public class PasswordServiceImpl implements PasswordService {
 
     @Override
     public String generateSalt() {
-        // BCrypt已包含盐值，此方法不再需要
+        // 兼容历史表结构，salt 字段仍要求非空
         return "";
     }
 
@@ -44,7 +44,7 @@ public class PasswordServiceImpl implements PasswordService {
     public void setPassword(AdminUser user, String rawPassword) {
         // 直接使用BCrypt加密密码
         user.setPassword(passwordEncoder.encode(rawPassword));
-        // 不再需要设置盐值
-        user.setSalt(null);
+        // BCrypt 已内置盐值，这里仅写入空串以兼容非空字段约束
+        user.setSalt(generateSalt());
     }
 }