Add unread message indicators for rooms (#38)

Shows a dot on rooms with unread messages in the sidebar, updated live
over WebSocket. Reuses the same offline-member audience computation
already used for push notifications: a member gets the real-time signal
whenever they aren't currently connected to that room's channel, which
correctly covers both "room not open" and "room open but tab
backgrounded" (the client leaves a room's channel while hidden).

Persisted server-side via a new room_memberships.last_read_at column so
state survives reload and stays consistent across devices, advanced by
an explicit mark-read call the frontend makes on room-open and on each
live message received while the room is genuinely visible -- gated on a
live visibility check, not a cached ref, so a backgrounded-but-open room
keeps accumulating unread instead of auto-marking-read the instant a
message arrives somewhere it can't be seen.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/api/rooms.ts

@@ -59,6 +59,10 @@ export function listRoomAttachments(roomId: string): Promise<RoomAttachment[]> {
   return apiFetch<RoomAttachment[]>(`/api/rooms/${roomId}/attachments`)
 }
 
+export function markRoomRead(roomId: string): Promise<void> {
+  return apiFetch<void>(`/api/rooms/${roomId}/read`, { method: 'POST' })
+}
+
 export function addRoomMember(roomId: string, userId: string): Promise<RoomMember> {
   return apiFetch<RoomMember>(`/api/rooms/${roomId}/members`, {
     method: 'POST',