feat(backup): prefer browser request host for backup filename

Name downloaded DB backups after the host shown in the panel title (c.Request.Host) when available, falling back to the configured web domain and then the public IP. Telegram-sent backups have no request context and keep the domain/IP behavior.

internal/web/controller/server.go

@@ -298,7 +298,7 @@ func (a *ServerController) getDb(c *gin.Context) {
 		return
 	}
 
-	filename := a.serverService.BackupFilename()
+	filename := a.serverService.BackupFilename(c.Request.Host)
 	if !filenameRegex.MatchString(filename) {
 		c.AbortWithError(http.StatusBadRequest, fmt.Errorf("invalid filename"))
 		return