feat(guild): channel membership + public visibility

- new channel_members table; creator always added, plus selected members
- Channel.isPublic (default false): public channels visible to all guild
  members; non-public only to explicit members
- GET /channels filters to channels visible to the requesting user

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

src/entities/channel.entity.ts

@@ -19,6 +19,11 @@ export class Channel {
   @Column({ type: 'boolean', default: false })
   isPrivate!: boolean;
 
+  // public channels are visible to every guild member (including those who
+  // join after the channel was created); default off (unchecked)
+  @Column({ type: 'boolean', default: false })
+  isPublic!: boolean;
+
   @Index()
   @Column({ default: 0 })
   lastSeq!: number;