Implement `GET /campaigns/:id/donations` endpoint

[ayshadogo]

User Story:
As a visitor, I want to see a campaign's donation history so that I can view the donor leaderboard.
Acceptance Criteria:

• Returns paginated donations list (20 per page)
• Anonymous donations show wallet as "Anonymous"
• Sorted by amount descending by default
• Includes: rank, walletAddress, amount, assetCode, createdAt, txHash

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