[BE] update leveling up system

[rgodha24]

Description

Right now, in src/services/pets.ts line 103, we just hardcode the amount of xp for each level (with the constnat LEVEL_THRESHOLD in src/utils/constants.ts

can you update this to be like the notion: https://www.notion.so/gtbitsofgood/Coin-Logic-293bd5d1ba1580f0a7aef3315d49d5e5

Acceptance Criteria

• update xp logic

Other Notes

[rgodha24]

This is partially completed, with the streak system left for Spring 2026 as described in the transition guide