test: add invite code validation tests and fix migration rollback

- Add TestInviteCodeSignup integration tests for valid/invalid invite codes
- Fix migration downgrade: avoid dropping trigger dependency
- Add DB CHECK constraint for invite_codes.code format
- Update runtime-route.md with invite_code documentation
- Update runtime-runbook.md with change log

docs/runtime/runtime-route.md

@@ -20,7 +20,8 @@
   "username": "string (3-30 chars)",
   "email": "string (email)",
   "password": "string (min 6 chars)",
-  "redirect_to": "string? (optional)"
+  "redirect_to": "string? (optional)",
+  "invite_code": "string? (8 chars, 排除易混淆字符 0/1/I/L/O)"
 }
 ```
 
@@ -31,6 +32,12 @@
 }
 ```
 
+**邀请码说明：**
+- 可选字段，不填则注册不受影响
+- 格式：8 位字母数字组合，排除易混淆字符 (0, 1, I, L, O)
+- 注册时传入有效邀请码会建立邀请关系并增加邀请码使用次数
+- 无效邀请码（不存在/已禁用/已过期/已达上限）不会阻断注册成功
+
 **Errors:**
 - 422: 请求参数无效
 - 429: 请求过于频繁