feat(guild): closed channel (discussion-complete support)

Channel.closed; POST /channels/:id/close (member-only); message/command
posts on closed channel -> 409 {error:channel_closed}; GET history still
allowed; listForUser carries closed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/messaging/messaging.controller.ts

@@ -1,5 +1,6 @@
 import {
   Body,
+  ConflictException,
   Controller,
   Delete,
   Get,
@@ -143,6 +144,9 @@ export class MessagingController {
 
     const channel = await this.channelRepo.findOne({ where: { id: channelId } });
     if (!channel) throw new NotFoundException('channel not found');
+    if (channel.closed) {
+      throw new ConflictException({ error: 'channel_closed', message: 'channel is closed' });
+    }
     const xType = channel.xType ?? 'general';
     const isRotating = xType === 'discuss' || xType === 'work';
     const authorUserId = String(body.authorUserId ?? 'anonymous');