feat(stellar): Implement subscription/recurring payments

[zeemscript]

Description

Implement a subscription system that supports recurring payments on the Stellar network. This feature will enable monthly and yearly subscription plans with automatic payment processing and status tracking.

Acceptance Criteria

• Support for monthly and yearly subscription intervals
• Track subscription status (active, paused, cancelled, expired)
• Automatic payment processing at subscription intervals
• Subscription management endpoints (create, update, cancel, renew)

Files to modify/create

• src/modules/stellar/subscription.service.ts
• src/modules/stellar/subscription.controller.ts
• src/modules/stellar/entities/subscription.entity.ts
• src/modules/stellar/dto/subscription.dto.ts