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/entities/channel.entity.ts

@@ -31,6 +31,10 @@ export class Channel {
   @Column({ type: 'boolean', default: false })
   isPublic!: boolean;
 
+  // closed (e.g. discussion-complete): history readable, new posts rejected
+  @Column({ type: 'boolean', default: false })
+  closed!: boolean;
+
   @Index()
   @Column({ default: 0 })
   lastSeq!: number;