feat(mailbox): 支持用户自助申请邮箱

- 后端新增邮箱申请配置与接口，限制可申请域名并校验保留前缀。
- 管理后台系统设置增加自助申请邮箱开关、开放域名和禁止前缀配置。
- 个人中心新增申请邮箱入口，并在邮箱页优化当前邮箱切换与空列表处理。
- 补充相关测试与环境变量示例配置。

apps/api/internal/app/app.go

@@ -627,6 +627,14 @@ func defaultFolderDefs() []struct{ name, role string } {
 }
 
 func (a *App) createMailbox(ctx context.Context, userID, domainID, localPart, displayName, password string, quotaMB int, status string) (string, error) {
+	passwordHash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		return "", err
+	}
+	return a.createMailboxWithPasswordHash(ctx, userID, domainID, localPart, displayName, string(passwordHash), quotaMB, status)
+}
+
+func (a *App) createMailboxWithPasswordHash(ctx context.Context, userID, domainID, localPart, displayName, passwordHash string, quotaMB int, status string) (string, error) {
 	localPart = normalizeLocalPart(localPart)
 	if localPart == "" {
 		return "", errors.New("invalid local part")
@@ -642,10 +650,6 @@ func (a *App) createMailbox(ctx context.Context, userID, domainID, localPart, di
 		return "", err
 	}
 	address := localPart + "@" + domain
-	passwordHash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
-	if err != nil {
-		return "", err
-	}
 	if displayName == "" {
 		displayName = address
 	}
@@ -659,7 +663,7 @@ func (a *App) createMailbox(ctx context.Context, userID, domainID, localPart, di
 	id := newID("mbx")
 	now := a.now().UTC().Format(time.RFC3339Nano)
 	_, err = tx.ExecContext(ctx, `INSERT INTO mailboxes(id,user_id,domain_id,local_part,address,display_name,password_hash,quota_mb,status,created_at,updated_at)
-		VALUES(?,?,?,?,?,?,?,?,?,?,?)`, id, userID, domainID, localPart, address, displayName, string(passwordHash), quotaMB, status, now, now)
+		VALUES(?,?,?,?,?,?,?,?,?,?,?)`, id, userID, domainID, localPart, address, displayName, passwordHash, quotaMB, status, now, now)
 	if err != nil {
 		return "", err
 	}