chore(deploy): 优化 split stack 的 SMTP 配置

- 为 `docker-compose.stack.yml` 的 API 容器默认注入 `LANQIN_SMTP_HOST/PORT`，避免 Webmail 发信配置漂移。
- 补充 `.env.example` 和 `README`，说明 split stack 默认走 `postfix`，以及外部 SMTP 的覆盖方式。
- 新增 `sent_message_dedupe_keys` 表和相关读写逻辑，确保已发送消息去重可回收，并补充对应测试。

apps/api/internal/app/app.go

@@ -233,6 +233,13 @@ func (a *App) migrate(ctx context.Context) error {
 		`CREATE INDEX IF NOT EXISTS idx_messages_search ON messages(mailbox_id, subject, from_addr, from_name, snippet)`,
 		`CREATE UNIQUE INDEX IF NOT EXISTS idx_messages_mailbox_raw_path ON messages(mailbox_id, raw_path) WHERE raw_path <> '' AND mailbox_id IS NOT NULL`,
 		`CREATE UNIQUE INDEX IF NOT EXISTS idx_messages_unregistered_raw_path ON messages(raw_path) WHERE raw_path <> '' AND mailbox_id IS NULL`,
+		`CREATE TABLE IF NOT EXISTS sent_message_dedupe_keys (
+			mailbox_id TEXT NOT NULL REFERENCES mailboxes(id) ON DELETE CASCADE,
+			folder_id TEXT NOT NULL REFERENCES folders(id) ON DELETE CASCADE,
+			message_id TEXT NOT NULL,
+			created_at TEXT NOT NULL,
+			PRIMARY KEY(mailbox_id, folder_id, message_id)
+		)`,
 		`CREATE TABLE IF NOT EXISTS attachments (
 			id TEXT PRIMARY KEY,
 			message_id TEXT NOT NULL REFERENCES messages(id) ON DELETE CASCADE,