Implement `POST /donations` record donation endpoint

[ayshadogo]

User Story:
As a donor, I want my donation recorded on the backend after a successful blockchain transaction so that it appears in campaign stats.
Acceptance Criteria:

• Accepts: campaignId, txHash, amount, assetCode, assetIssuer, isAnonymous
• Verifies transaction on Stellar Horizon before recording
• Prevents duplicate recording (idempotent on txHash)
• Returns created donation record
• Triggers campaign stats recalculation

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