Implement webhook delivery with retry and ordering guarantees

[grantfox-oss]

src/config.rs accepts WEBHOOK_URLS but there is no implementation of webhook delivery. Without webhook dispatch, external systems cannot react to registration and revocation events in real time. There is no retry strategy, no ordering guarantee across multiple webhooks, and no dead-letter queue for failed deliveries.

Evidence:

• src/config.rs parses WEBHOOK_URLS from environment.
• src/event.rs defines webhook event structures but they are not sent.
• No integration between contract events or Soroban event parsing and webhook transmission.
• No exponential backoff, jitter, or dead-letter queue implementation.

Impact:

External systems miss critical events or receive them out of order, leading to stale replicas and inconsistent user experience. Failed webhooks corrupt external database state if retried naively.

Likely files to change:

• src/event.rs
• src/config.rs
• src/stellar.rs
• src/metrics.rs
• src/cache.rs
• Cargo.toml
• README.md
• New webhook dispatch module or service

Acceptance criteria:

• Webhook events are dispatched asynchronously after event finalization.
• Failed deliveries trigger exponential backoff with jitter.
• Webhook delivery includes idempotency tokens based on transaction hash and event index.
• Dead-letter queue captures failed deliveries for manual replay.
• Multiple webhook recipients are contacted in order without skipping.
• Metrics track delivery latency, failure rate, and dead-letter queue depth.
• README documents webhook event schema and retry semantics.

[feyishola]

Hi im Feyishola a software developer with over 4 years of experience. pls i would love to work on this

[grantfox-oss]

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

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #28)
2. Your PR will be reviewed by the Proof-Stell 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 campaign!

@feyishola's PR #42 was approved and merged by @MaryammAli.

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

👏 Great work, @feyishola! Keep contributing to Proof-Stell.