Implement Rewards Module with Stella Token Distribution

[Mkalbani]

Description

Create a rewards module that manages stella-based token rewards distributed upon puzzle completion. Rewards are tied to difficulty and performance, serving as both in-game currency and a potential on-chain asset. The module needs clear rules for when rewards are granted, how they accumulate, and how they can be queried.

Acceptance Criteria

• A rewards module is scaffolded at src/rewards/
• Reward entity includes id, userId, sessionId, type (stella), amount, and grantedAt
• Rewards are granted automatically on session completion
• Reward amount scales with puzzle difficulty and score achieved
• GET /rewards/me returns the authenticated user's reward history and total balance
• GET /rewards/balance returns a single total stella balance for the user
• Reward duplication is prevented per session
• Unit tests cover reward calculation, grant, balance query, and duplication prevention

[chuks68]

hello maintainer, please assign me to this issue
i am confident i can resolve this issue and have a pr ready in 4-5 hours

[grantfox-oss]

🦊 GrantFox — @chuks68 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #34)
2. Your PR will be reviewed by the MindFlowInteractive maintainers

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