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-member.entity.ts

@@ -0,0 +1,19 @@
+import { Column, CreateDateColumn, Entity, Index, PrimaryGeneratedColumn } from 'typeorm';
+
+@Entity('channel_members')
+@Index(['channelId', 'userId'], { unique: true })
+@Index(['userId'])
+export class ChannelMember {
+  @PrimaryGeneratedColumn('uuid')
+  id!: string;
+
+  @Index()
+  @Column({ type: 'char', length: 36 })
+  channelId!: string;
+
+  @Column({ type: 'varchar', length: 64 })
+  userId!: string;
+
+  @CreateDateColumn()
+  createdAt!: Date;
+}