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