Dedicated payload column for system messages

[codebestia]

messages.ciphertext is the only content column on the messages table. System messages (device add/revoke, membership changes) are produced by JSON.stringify({userId, change}) and written directly into that same field (emitDeviceChangeEvent in routes/devices.ts). This conflates genuine E2EE ciphertext (opaque, per-device-encrypted) with structured unencrypted metadata in one column.

Acceptance criteria:

• A new dedicated jsonb column (systemPayload) is added to messages for content_type = 'system' rows, with a migration
• emitDeviceChangeEvent and any other system-message producer write to the new column, leaving ciphertext null for system messages
• Read paths read systemPayload for system messages instead of JSON-parsing ciphertext
• A check constraint ensures content_type = 'system' rows have ciphertext IS NULL and non-null systemPayload

[doctorlight0]

@doctorlight0 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I've reviewed the issue and understand the implementation requirements. I'll ensure the solution is well-structured, maintainable, and consistent with the project's conventions. I'd like to contribute by working on this issue.

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[theo372001]

@theo372001 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello maintainer please let me work on this for you.

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[promisszn]

@promisszn has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi i'd like to work on this

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