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/database.config.ts

@@ -1,6 +1,7 @@
 import { TypeOrmModuleOptions } from '@nestjs/typeorm';
 import { Guild } from './entities/guild.entity';
 import { Channel } from './entities/channel.entity';
+import { ChannelMember } from './entities/channel-member.entity';
 import { Message } from './entities/message.entity';
 import { DmConversation } from './entities/dm-conversation.entity';
 import { DmParticipant } from './entities/dm-participant.entity';
@@ -19,6 +20,7 @@ export const buildTypeOrmConfig = (): TypeOrmModuleOptions => ({
   entities: [
     Guild,
     Channel,
+    ChannelMember,
     Message,
     DmConversation,
     DmParticipant,