Task #43: Stop counting system messages as unread

Problem: Group rename, member-add, and member-remove events insert rows
into the messages table with the actor as senderId, which made them
increment the unread badge for everyone else just like a normal chat
message. That created notification noise for routine admin tweaks.

Fix: In `artifacts/api-server/src/routes/conversations.ts`, the
`buildConversationDetails` unread count query now restricts to
`messages.kind = 'user'`, so only real user chat messages contribute to
the unread count returned for the conversation list / bell badge.

Notes / non-changes:
- The conversation-list "last message" preview is intentionally left
  unfiltered, so the most recent activity (including system messages)
  still surfaces in the list — matches the task's "Done looks like".
- Push-style chat notifications (`createMessageNotifications`) are only
  invoked from the user send-message route, never from
  `insertAndEmitSystemMessage`, so no additional guard is required for
  system events today.
- Frontend chat.tsx already renders system messages distinctly and
  shows them in the list preview; no UI change needed.

Replit-Task-Id: 3734d73b-2df7-4f32-9655-6c7a9e23536c

artifacts/api-server/src/routes/conversations.ts

@@ -116,6 +116,7 @@ async function buildConversationDetails(conversationId: number, currentUserId: n
     .where(
       and(
         eq(messagesTable.conversationId, conversationId),
+        eq(messagesTable.kind, "user"),
         sql`${messagesTable.id} NOT IN (
           SELECT message_id FROM message_reads WHERE user_id = ${currentUserId}
         )`,