Pull Request: Support for "Gifted" Subscriptions (#57, #14)

[JerryIdoko]

📝 Description
This PR refactors the core subscription logic to decouple the account providing the funds (Payer) from the account receiving the service (Beneficiary). This enables "Gifting" workflows where a sponsor can pay for another user's access while maintaining control over the financial lifecycle of the stream.

🎯 Key Changes
Struct Refactor: Updated the Subscription struct to include both payer_address and beneficiary_address.

Permission Logic: * Payer Only: Can fund, top-up, or cancel the subscription stream.

Beneficiary Only: Recognized by the content-gating middleware as the "Active" subscriber.

New Storage Mapping: Implemented a GiftsReceived map (Map<Address, Vec>) to allow beneficiaries to query and display all active sponsorships on their personal dashboard.

Middleware Compatibility: Updated the is_active_subscriber view function to check the beneficiary field, ensuring seamless integration with existing gated-access tools.

💻 Implementation Detail: Data Structure Update
The core state change involves shifting from a single owner to a dual-address model:

Rust
pub struct Subscription {
pub id: u64,
pub payer: Address, // The entity paying the XLM/Asset
pub beneficiary: Address, // The entity receiving access
pub start_time: u64,
pub end_time: u64,
pub rate_per_sec: i128,
}
✅ Acceptance Criteria Checklist
[x] Separation of Concerns: Verified that payer controls the money and beneficiary gets the access.

[x] Dashboard Support: The new GiftsReceived mapping correctly indexes subscriptions by the beneficiary's address.

[x] Security: Only the payer can trigger a cancel_subscription call to prevent beneficiaries from "stealing" or prematurely ending a stream they don't fund.

[x] Gating Logic: The access-check function now correctly validates the beneficiary address against the current ledger timestamp.

🚀 How to Verify
Unit Tests:

Bash
cargo test test_gift_subscription_access
cargo test test_payer_only_cancellation
Storage Inspection:
Deploy to a local sandbox and use soroban contract read to verify the GiftsReceived entry for a test beneficiary address.

🔗 Linked Issues
Closes #57
Closes #14

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