And since the server provides both seeds, then the game is not provably fair because those seeds could be tested before the runs to warrant a looser seed for the long run.
To make it a provably fair game, users should be able to change the client seed before the bet.
So you're saying the server could get a seed designed to only give "heads" or "tails"? That's possible, but if the server tries to cheat that way, you can also use it at your advantage so that makes it a risky game to play.
We can use it for our advantage but would be complex to survive in the long run with a seed that will win 150 flips from 500, if we push our luck we will find the point in the seed where 350 from 500 will win, But that would be a rude start.
It would be nice if OP could add a bet verification tool on his site and a page with full information about the bot.