Stream messaging integration: treat delivery and moderation as data

Messages can be edited, deleted, retried, moderated, and received out of order.

• Use stable channel, message, and user IDs.
• Verify webhook events and process them idempotently.
• Preserve event time and receipt time.
• Enforce membership and permissions server-side.
• Apply retention and moderation consistently to attachments.

Test reconnects, offline send, unread counts, thread replies, bans, deleted messages, and rate limits. Keep message bodies out of routine logs and provide a clear recovery state when synchronization fails.