[P2] Channels: Off-chain state update signing and exchange protocol

[Calebux]

Parent Issue

#825

Summary

Implement the off-chain protocol for exchanging signed balance updates between user and renewal executor during each billing cycle.

Details

• Each state update = { channel_id, balance_user, balance_executor, sequence_number }
• Both parties sign the state update (user via Freighter, executor via backend key)
• State updates are exchanged via a simple HTTP API (or WebSocket for real-time)
• Both parties persist the latest signed state locally
• Sequence number monotonically increases — higher seq always supersedes lower

Affected Code

• New: backend/src/services/channel-state.ts — state management + signing
• New: backend/src/routes/channels.ts — HTTP endpoints for state exchange
• client/lib/ — new payment-channel.ts client
• DB: new payment_channels and channel_states tables

Acceptance Criteria

• Both parties can sign and exchange state updates
• Latest state is persisted and recoverable
• Invalid signatures are rejected
• Sequence number ordering is enforced

[Chi-nenye1975]

@Chi-nenye1975 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, I'd love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Chi-nenye1975 to this issue.

[daveedAJ]

@daveedAJ has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Team,

I'd like to work on this issue. I'll implement the off-chain channel state exchange flow, including signing, validation, persistence, sequence handling, and the required backend/client endpoints.

Thanks for considering me.

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

[drips-wave]

Congratulations, @Chi-nenye1975! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Chi-nenye1975: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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