docs(email): treat SMTP as an email backend in self-host docs and startup warning

Address review feedback on #2666:

- server: startup warning now fires only when both RESEND_API_KEY and SMTP_HOST
  are empty, since either one is a valid email backend. Otherwise the log
  mis-tells SMTP-only operators that verification codes go to stdout.
- self-host-quickstart (EN/ZH): tell readers to fetch the verification code
  from whichever backend they configured (Resend or SMTP); fall back to
  stdout only when neither is configured.
- auth-setup (EN/ZH): \"without Resend\" → \"without any email backend
  configured\" so the wording stays correct now that SMTP is a first-class
  option.

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

apps/docs/content/docs/auth-setup.mdx

@@ -51,7 +51,7 @@ STARTTLS is upgraded automatically when the server advertises it. Port 465 (SMTP
 
 The old behavior where non-production instances accepted `888888` by default has been removed. Unless you explicitly configure it, typing `888888` is treated like any other wrong code.
 
-Local development without Resend should use the generated code printed in server logs. If you need deterministic local/private automation, set `MULTICA_DEV_VERIFICATION_CODE` to a 6-digit value such as `888888`, and keep `APP_ENV` non-production:
+Local development without any email backend configured (no Resend, no SMTP) should use the generated code printed in server logs. If you need deterministic local/private automation, set `MULTICA_DEV_VERIFICATION_CODE` to a 6-digit value such as `888888`, and keep `APP_ENV` non-production:
 
 ```bash
 APP_ENV=development

apps/docs/content/docs/auth-setup.zh.mdx

@@ -51,7 +51,7 @@ RESEND_FROM_EMAIL=noreply@yourdomain.com  # 同时作为 SMTP From: 头
 
 旧版「非 production 默认接受 `888888`」的行为已经移除。除非你显式配置，否则输入 `888888` 会和普通错误验证码一样被拒绝。
 
-不配 Resend 的本地开发，应使用 server 日志里打印的随机验证码。如果你需要确定性的本地/私有自动化测试，可以把 `MULTICA_DEV_VERIFICATION_CODE` 设成一个 6 位数字，比如 `888888`，并保持 `APP_ENV` 为非 production：
+没配任何邮件后端（Resend 和 SMTP 都没设）的本地开发，应使用 server 日志里打印的随机验证码。如果你需要确定性的本地/私有自动化测试，可以把 `MULTICA_DEV_VERIFICATION_CODE` 设成一个 6 位数字，比如 `888888`，并保持 `APP_ENV` 为非 production：
 
 ```bash
 APP_ENV=development

apps/docs/content/docs/self-host-quickstart.mdx

@@ -93,7 +93,7 @@ For more auth configuration (OAuth, signup allowlist) and the full SMTP variable
 Open [http://localhost:3000](http://localhost:3000):
 
 - Enter your email
-- Grab the verification code from the Resend email (or, if you haven't configured Resend, from the server container stdout — look for the `[DEV] Verification code` line)
+- Grab the verification code from your configured email backend (Resend or SMTP relay); if neither is configured, copy it from the server container stdout — look for the `[DEV] Verification code` line
 - Do not use `888888` unless you explicitly set `MULTICA_DEV_VERIFICATION_CODE=888888` on a non-production private instance
 - Log in and create your first workspace
 
@@ -122,7 +122,7 @@ Same flow as Cloud — see [Cloud quickstart → Steps 5-6](/cloud-quickstart#5-
 ## Common issues
 
 - **Backend won't start**: check container logs with `docker compose -f docker-compose.selfhost.yml logs backend`; usually it's a bad `DATABASE_URL` or `JWT_SECRET` in `.env`
-- **Verification code not received**: Resend isn't configured → look for `[DEV] Verification code` in `docker compose logs backend`
+- **Verification code not received**: no email backend is configured (neither Resend nor SMTP) → look for `[DEV] Verification code` in `docker compose logs backend`
 - **WebSocket won't connect**: for public deployments you must set `FRONTEND_ORIGIN` to your real frontend domain; see [Troubleshooting → WebSocket won't connect](/troubleshooting#websocket-wont-connect)
 
 ## Next steps

apps/docs/content/docs/self-host-quickstart.zh.mdx

@@ -92,7 +92,7 @@ RESEND_FROM_EMAIL=noreply@yourdomain.com  # 同时作为 SMTP From: 头
 打开 [http://localhost:3000](http://localhost:3000)：
 
 - 输入你的邮箱
-- 从 Resend 邮件里拿验证码（或者前面没配 Resend 的话从 server 容器的 stdout 里抄 `[DEV] Verification code` 这行）
+- 从你配置的邮件后端（Resend 或 SMTP relay）收到的邮件里拿验证码；两者都没配的话，从 server 容器的 stdout 里抄 `[DEV] Verification code` 这行
 - 不要直接使用 `888888`；只有在非 production 私有实例上显式设置 `MULTICA_DEV_VERIFICATION_CODE=888888` 后它才会生效
 - 登录后创建第一个工作区
 
@@ -121,7 +121,7 @@ multica setup self-host
 ## 常见问题
 
 - **后端起不来**：看容器日志 `docker compose -f docker-compose.selfhost.yml logs backend`；常见是 `.env` 里 `DATABASE_URL` 或 `JWT_SECRET` 有问题
-- **验证码收不到**：没配 Resend → 从 `docker compose logs backend` 里找 `[DEV] Verification code`
+- **验证码收不到**：没配任何邮件后端（Resend 和 SMTP 都没设） → 从 `docker compose logs backend` 里找 `[DEV] Verification code`
 - **WebSocket 连不上**：公网部署必须设 `FRONTEND_ORIGIN` 成你真实的前端域名；见 [故障排查 → WebSocket 连不上](/troubleshooting#websocket-连不上)
 
 ## 下一步

server/cmd/server/main.go

@@ -123,8 +123,8 @@ func main() {
 	if os.Getenv("JWT_SECRET") == "" {
 		slog.Warn("JWT_SECRET is not set — using insecure default. Set JWT_SECRET for production use.")
 	}
-	if os.Getenv("RESEND_API_KEY") == "" {
-		slog.Warn("RESEND_API_KEY is not set — email verification codes will be printed to the log instead of emailed.")
+	if os.Getenv("RESEND_API_KEY") == "" && strings.TrimSpace(os.Getenv("SMTP_HOST")) == "" {
+		slog.Warn("no email backend configured (RESEND_API_KEY and SMTP_HOST both empty) — verification codes will be printed to the log instead of emailed.")
 	}
 	if os.Getenv("MULTICA_DEV_VERIFICATION_CODE") != "" {
 		if strings.EqualFold(strings.TrimSpace(os.Getenv("APP_ENV")), "production") {