Conversation + user rooms as a performance layer

[codebestia]

Implement room:conversation:{conversationId} (typing, receipts, presence, fanout optimization) and room:user:{userId} (cross-device account events). Rooms are ephemeral and rebuildable from Postgres+Redis on restart. They optimize fanout but the authoritative delivery decision still validates conversation_members.

Acceptance criteria:

• Rooms reconstructable after a gateway restart
• Typing/receipts use rooms; message delivery still verifies membership
• No security decision relies solely on room membership