Add dispute events, subscription engine, batch verification, and relative stream creation

[success-OG]

closes #108
closes #109
closes #110
closes #128

Summary

Implements 4 new features for the FluxaPay payment processor contract.

Features

1. Fee Structure Customization

• Added FeeConfig struct with configurable platform fee (bps), fixed fee, and custom fee recipient
• Merchants can now configure transaction fees via register_merchant and update_merchant
• Added calculate_fee and get_fee_recipient helper functions for fee computation

2. Payment Metadata Support

• Added metadata: Option<Map<String, String>> field to PaymentLink
• Merchants can attach arbitrary key-value metadata to payment links
• Metadata is accessible upon payment completion

3. Atomic Settlement & Swap

• Created DexRouter interface for DEX integration (Soroswap-compatible)
• Added swap_and_pay function for atomic token swap and payment creation
• Supports custom DEX router address and configurable swap paths

4. Recipient Address Rotation

• Added RecipientStream struct for streaming payment management
• Implemented update_recipient with recipient authentication
• Updates RecipientStreams index atomically
• Emits RecipientUpdated event for transparency
• Added create_recipient_stream and claim_from_stream for streaming payments

Files Changed

• fluxapay/src/merchant_registry.rs - FeeConfig, fee_config field, updated functions
• fluxapay/src/payment_link.rs - metadata field, updated create_link
• fluxapay/src/dex_router.rs - New DEX router interface
• fluxapay/src/lib.rs - swap_and_pay, update_recipient, RecipientStream, PaymentConfig

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