feat(smart-contract): Implement Agent Subscription and Recurring Payment Model

[devJaja]

Summary

Add subscription-based payment model where clients can subscribe to agent services with recurring XLM payments and automatic renewal.

Motivation

Pay-per-task is not always optimal. Subscriptions provide predictable costs for clients and stable income for agents.

Acceptance Criteria

• create_subscription function with duration, payment amount, and agent
• Subscription stored with start time, end time, and payment schedule
• renew_subscription extends duration with new payment
• cancel_subscription with prorated refund
• check_subscription verifies if subscription is active
• Auto-renewal support (opt-in)
• Events: SubscriptionCreated, SubscriptionRenewed, SubscriptionCancelled, PaymentProcessed
• Unit tests for create, renew, cancel, and expiry

Files to Modify

File	Action
smart-contracts/contracts/agent_registry/src/lib.rs	Add subscription functions
smart-contracts/contracts/agent_registry/src/types.rs	Add Subscription struct
smart-contracts/contracts/agent_registry/src/errors.rs	Add subscription error codes
smart-contracts/contracts/agent_registry/src/events.rs	Add subscription events
smart-contracts/contracts/agent_registry/src/test.rs	Add subscription tests

How to Contribute

1. Create branch feat/agent-subscriptions
2. Design subscription storage structure
3. Implement create/renew/cancel functions
4. Add prorated refund calculation
5. Write comprehensive unit tests
6. Run cargo test and cargo clippy before submitting PR

[nasarajoe]

kindly assign me

[grantfox-oss]

🦊 GrantFox — @nasarajoe has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #258)
2. Your PR will be reviewed by the Epta-Node maintainers

Good luck! Track your progress on GrantFox.

[spiffamani]

📋 Grainlify Application

@spiffamani has applied to work on this issue as part of the Grainlify program.

hi, i am a full stack engineer i can work on this

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Repo Maintainers: To accept this application, review their application or assign @spiffamani to this issue.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@nasarajoe's PR #457 was approved and merged by @devJaja.

🏆 @nasarajoe: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (366 total points). Track your full progress on GrantFox.

👏 Great work, @nasarajoe! Keep contributing to Epta-Node.