feat(server): pro 套餐设备上限 5 → 3

- seed(sqlite/mysql 000007)+ 001_init:pro max_devices 5→3(全新库直接对)
- 新增迁移 000019(sqlite/mysql,up/down):UPDATE plans SET max_devices=3
  WHERE code='pro' —— 已迁移的线上库靠它更新;down 回退 5
- 000002/001 注释同步(free 1 / pro 3 / team 10)
- 测试:sqlite_migrate 期望版本 18→19;devices 集成测试 pro 断言 5→3
- free(1)/team(10)不变

注:设备上限当前仍未强制执行(登录注册 MaxDevices=0),仅数据/展示口径;
真正启用+超限 UX 见 todo #16。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/migrations/001_init.sql

@@ -25,14 +25,14 @@ CREATE TABLE IF NOT EXISTS users (
 CREATE TABLE IF NOT EXISTS plans (
   id            BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
   code          ENUM('free','pro','team') NOT NULL UNIQUE,
-  max_devices   INT  NOT NULL,              -- free 1 / pro 5 / team 10
+  max_devices   INT  NOT NULL,              -- free 1 / pro 3 / team 10
   daily_minutes INT  NULL,                  -- free 10; NULL = unlimited
   ad_gate       BOOLEAN NOT NULL DEFAULT FALSE  -- free users must watch an ad daily
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 
 INSERT IGNORE INTO plans (code, max_devices, daily_minutes, ad_gate) VALUES
   ('free', 1,  10,   TRUE),
-  ('pro',  5,  NULL, FALSE),
+  ('pro',  3,  NULL, FALSE),
   ('team', 10, NULL, FALSE);
 
 -- -------------------------------------------------------------------------