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/channels/channels.controller.ts

@@ -54,4 +54,11 @@ export class ChannelsController {
     if (!userId) throw new UnauthorizedException('missing user');
     return this.channelsService.leaveChannel(channelId, userId);
   }
+
+  @Post(':id/close')
+  close(@Req() req: AuthedRequest, @Param('id') channelId: string) {
+    const userId = req.userId ?? '';
+    if (!userId) throw new UnauthorizedException('missing user');
+    return this.channelsService.closeChannel(channelId, userId);
+  }
 }