Document the chat conversation data model decision

[MicrowaveDev]

Summary

• review whether private chats should reuse social Group, Post, PostsContents, and Content relations
• record the decision to model direct and multi-member chat as dedicated conversations while preserving the existing append-only ChatEvent log
• explain current sender/recipient attachment relations, delivery behavior, metadata boundaries, and infrastructure that should remain shared
• propose additive ChatConversation and ChatConversationMember adoption without rewriting existing events
• link the review from module discovery and add the conversation aggregate before MLS persistence in the active TODO

Verification

• deterministic browser-first-chat-e2ee TODO context passed
• documentation links resolve to existing files
• staged diff and whitespace checks passed
• application tests were not rerun because this change is documentation-only