feat: add campaign clone endpoint and points-to-token redemption

[m1s0g1]

Closes #450
Closes #458

Full PR Description

Description:

Summary

This PR implements two features:

1. Issue Backend: Add campaign clone endpoint (duplicate existing campaign as new draft) #458: Campaign clone endpoint for duplicating campaigns
2. Issue Contracts: Add points-to-token redemption — exchange accumulated points for XLM or custom asset #450: Points-to-token redemption in rewards contract

Changes Made

Issue #458: Campaign Clone Endpoint

• ✅ Added POST /api/v1/campaigns/:id/clone endpoint (API key required)
• ✅ Clones all metadata: name (prefixed with 'Copy of'), description, tags, category, imageUrl, rewardPerAction
• ✅ New campaign gets: status: draft, new UUID, createdAt: now, null campaignContractId
• ✅ Fields NOT copied: contractId, startDate, endDate, participantCount, auditLogs
• ✅ Optional request body: { overrides: { name, description, ... } } to override specific fields
• ✅ Response includes clonedFrom: sourceId
• ✅ Audit log entry records: cloned_from: :sourceId
• ✅ Unit tests added and passing

Issue #450: Points-to-Token Redemption

• ✅ Added set_redemption_rate(env, admin, nonce, asset, rate_bps) — configure redemption rate
• ✅ Added redeem(env, user, points_amount) — burn points and transfer asset tokens
• ✅ Added redemption_rate(env) → (Address, u32) view function
• ✅ Added redemption_reserve(env) → u64 view function
• ✅ Added withdraw_reserve(env, admin, nonce, amount) — admin can reclaim assets
• ✅ Added fund_reserve(env, from, amount) — fund the redemption reserve
• ✅ Rejects redemption if reserve is insufficient (Error::InsufficientReserve)
• ✅ Emits redeemed event with topics (redeem, user) and data (points_burned, asset_amount)
• ✅ Uses SAC (Stellar Asset Contract) for token transfers

Testing

Backend Tests

cd backend
npm test

All clone tests pass:

• ✅ Clone creates new campaign with copied metadata
• ✅ Clone with overrides applies custom values
• ✅ Clone returns undefined for non-existent campaign
• ✅ Clone generates unique slug for cloned campaign

Contract Build

cd contracts/rewards
cargo build --release

✅ Contract compiles successfully

Checklist

Issue #458

• Add POST /api/v1/campaigns/:id/clone endpoint (API key required)
• Clone creates new campaign with copied metadata (name prefixed with 'Copy of')
• New campaign gets status: draft, new UUID, createdAt: now, null campaignContractId
• Fields NOT copied: contractId, startDate, endDate, participantCount, auditLogs
• Optional request body with overrides support
• Response includes clonedFrom: sourceId
• Audit log entry records cloned_from
• Unit tests added and passing

Issue #450

• Add set_redemption_rate function (admin only with nonce)
• Add redeem function (burns points, transfers assets)
• Add redemption_rate view function
• Add redemption_reserve view function
• Add withdraw_reserve function (admin only with nonce)
• Add fund_reserve function
• Reject redemption if reserve insufficient
• Emit redeemed event
• Use SAC for token transfers
• Contract compiles successfully

Closes #458
Closes #450

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