feat: add voting feature for challenges

[jatinbansal2705]

Upvote Challenge System

Adds a voting-based weekly challenge system to Spurti. Students spend SP to vote on proposed challenges. The challenge with the most SP wins and becomes active. Only voters of the winning challenge are eligible to participate; on completion they receive their invested SP back multiplied by the challenge's rewardMultiplier.

How it works

• Proposed challenges appear in a new Challenges tab in the student dashboard, each with a voting window, type (attendance / poll_participation), and reward multiplier.
• Students spend SP to vote. They can add more SP to an existing vote or withdraw (full refund) any time before voting closes.
• An admin resolves the round once voting ends. The challenge with the highest totalSpInvested transitions to active with a 7-day live window; all others are archived.
• Students who voted for the winning challenge are enrolled. On completion they earn spInvested * rewardMultiplier SP credited back via challenge_reward transactions.
• SP spent on losing challenges is forfeited. Withdrawals are only possible during the voting phase.

Endpoints

Method	Endpoint	Purpose
GET	/api/challenges/proposed	Open challenges + user's votes + SP balance
GET	/api/challenges/active	Currently active challenge + enrollment status
POST	/api/challenges/:id/vote	Cast or increment a vote ({ spPoints: number })
POST	/api/challenges/:id/withdraw	Withdraw vote, refund SP
POST	/api/admin/challenges/:id/resolve	Resolve round — pick winner, activate, archive losers

Seed data

Run node server/scripts/seedChallenges.js to create 3 sample proposed challenges with 7-day voting windows (idempotent — skips if challenges already exist).