feat(core): bind request services and implement registered tasks

Wire plugin services through Bind/InjectFrom and AppContext so HTTP and
workers resolve dependencies after Apply. Register TaskHandler objects
with persisted results, and implement send_email_code, mail:send,
cleanup_inactive_users, and dispatch_bot_msg.

backend/plugins/domain/user/models.go

@@ -109,6 +109,10 @@ type registerRequest struct {
 	Email    string `json:"email"`
 }
 
+type sendEmailCodeRequest struct {
+	Email string `json:"email" binding:"required"`
+}
+
 // changePasswordRequest 修改密码请求参数
 type changePasswordRequest struct {
 	OldPassword string `json:"old_password" binding:"required"`