新功能(#69): 支持用户级限速覆盖

Co-authored-by: multica-agent <github@multica.ai>

initialize/migrate/database/02153_user_subscribe_speed_limit.down.sql

@@ -0,0 +1,3 @@
+ALTER TABLE `user_subscribe`
+  DROP COLUMN IF EXISTS `speed_limit`,
+  DROP COLUMN IF EXISTS `traffic_limit`;

initialize/migrate/database/02153_user_subscribe_speed_limit.up.sql

@@ -0,0 +1,3 @@
+ALTER TABLE `user_subscribe`
+  ADD COLUMN IF NOT EXISTS `speed_limit` BIGINT NOT NULL DEFAULT 0 COMMENT 'User-level speed limit override (Mbps), 0 uses plan-level',
+  ADD COLUMN IF NOT EXISTS `traffic_limit` TEXT DEFAULT NULL COMMENT 'User-level traffic limit override (JSON), NULL uses plan-level';