Per-conversation monotonic `sequence_number` assignment

[codebestia]

Add lastSequenceNumber bigint to conversations (or a counter table) and assign each new message sequenceNumber = ++counter inside the same transaction that persists the message, using UPDATE ... RETURNING / row lock. Must be correct under concurrent senders and horizontal scaling.

Acceptance criteria:

• Sequence numbers are gapless and strictly increasing per conversation
• Concurrent sends do not produce duplicate sequence numbers
• Assignment happens atomically with message persistence

[shadrach68]

@shadrach68 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issue because I am a full-stack developer with 5+ years experience. I focus on identifying root causes and delivering clean, efficient, and maintainable solutions, with thorough testing to ensure reliable results.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @shadrach68 to this issue.

[drips-wave]

Congratulations, @shadrach68! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @shadrach68: 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

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊