feat(subscribe): add traffic limit rules and user traffic stats

- Add subscribe traffic_limit schema and migration\n- Support traffic_limit in admin create/update and list/details\n- Apply traffic_limit when building server user list speed limits\n- Add public user traffic stats API

initialize/migrate/database/02133_add_expired_node_group.down.sql

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+-- 回滚 user_subscribe 表的过期流量字段
+ALTER TABLE `user_subscribe`
+DROP COLUMN `expired_upload`,
+DROP COLUMN `expired_download`;
+
+-- 回滚 node_group 表的过期节点组字段
+ALTER TABLE `node_group`
+DROP INDEX `idx_is_expired_group`,
+DROP COLUMN `speed_limit`,
+DROP COLUMN `max_traffic_gb_expired`,
+DROP COLUMN `expired_days_limit`,
+DROP COLUMN `is_expired_group`;

initialize/migrate/database/02133_add_expired_node_group.up.sql

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+-- 为 node_group 表添加过期节点组相关字段
+ALTER TABLE `node_group`
+ADD COLUMN `is_expired_group` tinyint(1) NOT NULL DEFAULT 0 COMMENT 'Is Expired Group: 0=normal, 1=expired group' AFTER `for_calculation`,
+ADD COLUMN `expired_days_limit` int NOT NULL DEFAULT 7 COMMENT 'Expired days limit (days)' AFTER `is_expired_group`,
+ADD COLUMN `max_traffic_gb_expired` bigint DEFAULT 0 COMMENT 'Max traffic for expired users (GB)' AFTER `expired_days_limit`,
+ADD COLUMN `speed_limit` int NOT NULL DEFAULT 0 COMMENT 'Speed limit (KB/s)' AFTER `max_traffic_gb_expired`;
+
+-- 添加索引
+ALTER TABLE `node_group` ADD INDEX `idx_is_expired_group` (`is_expired_group`);
+
+-- 为 user_subscribe 表添加过期流量统计字段
+ALTER TABLE `user_subscribe`
+ADD COLUMN `expired_download` bigint NOT NULL DEFAULT 0 COMMENT 'Expired period download traffic (bytes)' AFTER `upload`,
+ADD COLUMN `expired_upload` bigint NOT NULL DEFAULT 0 COMMENT 'Expired period upload traffic (bytes)' AFTER `expired_download`;

initialize/migrate/database/02134_subscribe_traffic_limit.down.sql

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+-- Purpose: Rollback traffic_limit rules from subscribe
+-- Author: Claude Code
+-- Date: 2026-03-12
+
+-- ===== Remove traffic_limit column from subscribe table =====
+ALTER TABLE `subscribe` DROP COLUMN IF EXISTS `traffic_limit`;

initialize/migrate/database/02134_subscribe_traffic_limit.up.sql

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+-- Purpose: Add traffic_limit rules to subscribe
+-- Author: Claude Code
+-- Date: 2026-03-12
+
+-- ===== Add traffic_limit column to subscribe table =====
+SET @column_exists = (
+  SELECT COUNT(*)
+  FROM INFORMATION_SCHEMA.COLUMNS
+  WHERE TABLE_SCHEMA = DATABASE()
+    AND TABLE_NAME = 'subscribe'
+    AND COLUMN_NAME = 'traffic_limit'
+);
+
+SET @sql = IF(
+  @column_exists = 0,
+  'ALTER TABLE `subscribe` ADD COLUMN `traffic_limit` TEXT NULL COMMENT ''Traffic Limit Rules (JSON)'' AFTER `node_group_id`',
+  'SELECT ''Column traffic_limit already exists in subscribe table'''
+);
+
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;