Integrated 5V uninterruptible power supply (UPS) with a lithium cell?
What you want is not exactly simple. Most power banks are not "UPS", they are reserved storage, and usually operate either in charge mode (charging internal Li-Ion cell(s)), or in discharge mode, with external power disconnected. Many powerbanks have an explicit warning, "don't charge and discharge at the same time", see example.

More, since the only one cell is enough for your power backup requirements, but your in power source is 48V, you will need a two-step conversion, from 48V to something more manageable - I saw no ICs for single Li-Ion cells that can handle over 40 V input. 

To get "true UPS" functionality, your charger IC must have something called "Power Path management", or something along this line, like the Texas Instruments BQ24xxx/BQ25xxx family of chargers do. But in no case the IC will provide you with output voltage of 5V automatically, this is left for system designer to do. Therefore you are looking for overall architecture like the following one:



The smart-power-path IC will provide the smooth switch from external power to battery power, but you will need to manage the variable battery output using up- and down-converters by yourself. Just as this project suggests and implements.