Support multiple concurrent stakes per user

[Mosas2000]

Allow users to stake multiple times on same outcome:

• Accumulate stakes instead of overwrite
• Track stake history
• Show total staked over time

[Yusufolosun]

Hello @Mosas2000
I would love to take on this issue, I've reviewed the issue description and I can implement a maintainable solution fast.
I'm active and i respond and react to feedback fast