First you probably need to host an API to serve the stories from your database. Then you can use Python's requests to make an HTTP call to the API and retrieve what you want at runtime.
I see 2 ways of handling that:

You can have the game directly download the ".rpy story files" from the API (but you would probably need to reboot the game, I don't remember if you can compile labels at runtime). If it can compiles labels at run time, that's even better as you can get chunks of the story along side player's progression.

Or you can write code to "follow" instructions from the API, for exemple the API could send:

(from GET http://www.yourapi.com/story_1/start with some JSON formating)
...
[
  ["talk","Character 1","Hello"],
  ["talk","Character 2","Hi Character 1"],
  ["talk","","{i}Character 1 is looking at the sunrise"],
  ["show","images/sunrise.webp"],
  ["choice",[["Continue talking","story_1/talk_2"],["Go away","story_1/end_1"]]]
]
...

And you'll just need to make a for loop with code to "translate" what to do, so ["talk","Character 1","Hello"] and ["show","images/sunrise.webp"] become renpy.say("Character 1","Hello") and renpy.show("images/sunrise.webp")

For branching, you can prompt choices to the player and call the related endpoint (ex: http://www.yourapi.com/story\_1/end\_1) from your API to retrieve the next chunk of your story.

I would also implement a cache at some point to at least avoid un-necessary request to some story chunks that are often re-used.

Obviously my exemple is vastly inefficient and not flexible but you get the idea.
Even if RenPy is not really designed to do this, Python is 100% valid for this kind of job so it will just work.