System messages (`content_type='system'`) for membership + key events

[codebestia]

Add systemPayload jsonb nullable to messages. System messages carry structured JSON for events (member joined/left, device added/revoked, conversation renamed, MLS epoch change). These are NOT secret — they are routing/UX events. Add a check constraint ensuring systemPayload is null for non-system messages.

Acceptance criteria:

• systemPayload column added; only set when contentType='system'
• System events ordered inline with real messages
• Check constraint prevents systemPayload on non-system messages

[ojuotimi932]

@ojuotimi932 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainer can I resolve this kindly assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ojuotimi932 to this issue.

[drips-wave]

Congratulations, @ojuotimi932! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @ojuotimi932: 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.

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