feat(gateway,core): recurring/subscription payment flows

[aguilar1x]

Context

Connect currently supports one-off escrow checkouts only. Merchants with subscription business models (the primary use case for many SaaS/marketplace integrators) have no way to charge on a recurring schedule through Pacto.

Scope

• A subscription primitive in the gateway: create a recurring plan (amount, interval, asset) tied to a merchant + payer.
• Scheduled escrow creation on each billing cycle, reusing the quote engine (feat(gateway): OTC quote engine + signed quotes #18) for FX pricing at charge time.
• Webhook events for the subscription lifecycle: subscription.created, subscription.charged, subscription.failed, subscription.canceled.

Acceptance criteria

• A subscription created via the SDK produces a new escrow automatically on each cycle without manual intervention.
• A failed charge (e.g. insufficient funds) emits subscription.failed and does not silently retry indefinitely.
• Canceling a subscription stops future charges and emits subscription.canceled.

[Okorie2000-code]

Hi Maintainer,

I'd like to work on this issue. I have experience with backend systems, payment workflows, and SDK integrations, and I can submit a clean PR within 48 hours.

[abufauzahn]

Hi! I've carefully reviewed the entire task description and fully understand the requirements and expected outcome.
I have experience working on similar tasks and I'm confident I can complete this within about an hour. I'd be happy to take ownership of this issue and submit a clean, well-tested solution promptly.
If it's available, please assign it to me. Thank you!

[gabvega08]

Hi maintainers

Frontend Developer at Extreme Tech with 4+ years of experience creating modern, responsive, and user-focused web applications. Passionate about crafting intuitive interfaces, smooth interactions, and exceptional user experiences.

Currently exploring advanced UI animations, Three.js, and modern frontend technologies while continuously improving my skills in design and development.

[grantfox-oss]

🦊 GrantFox — @gabvega08 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #41)
2. Your PR will be reviewed by the PACTO-LAT maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@gabvega08's PR #48 was approved and merged by @aguilar1x.

🏆 @gabvega08: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (274 total points). Track your full progress on GrantFox.

👏 Great work, @gabvega08! Keep contributing to PACTO-LAT.