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where the game ends?

 and how i can work in restaurant after second Akira's trick?

Not sure if the next update is out yet but you cant work quite yet. You need to finish your training and run everyday until this section of the game ends, there will be some new cutscenes with a new girl and the story will continue from there. 

who's Sakuya?

(+1)

Im pretty sure Sakuya is the mom

She needs to pay debt, i have got 2455 Y, what i need to do?

Sakuya is daughter of restaurant owner, name of main character mom is Yasuka,

As story progresses the money you collected will be given automatically, Anyway there is no other way of making money other than collecting the hidden money in each place.

Okay, i collected all hidden money, what i need to do now? I can't find Sakuya

When you say, you have collected all money, then it means you should have unlocked cheat menu. (ie you got money from Minato's and ice cream parlour as well, which is available only after completing all the quest for Sakuya)
But if you havent completed the cheat menu and havent seen the minatos house, then you need to go check outside your home at night

i collected all hidden money at Minatos's and ice cream parlour, but cheat menu didn't unlock

I guess you didnt collect all money from Noriko's room and you can't go into her room after completig this far.