Migrate payment_proof uploads to R2 storage

### Overview
Updated all payment proof file handling to use Cloudflare R2 instead of local storage for better scalability and CDN performance

### Key Changes
- [Add detailed changes here]

### Technical Implementation
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### Impact
- [Add impact assessment here]

### Gambaran Umum
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### Perubahan Utama
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### Implementasi Teknis
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### Dampak
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