feat(guild): required channel x_type enum

Channel.x_type enum(general|work|report|discuss|triage|custom); required
and validated on channel creation (400 if missing/invalid).

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

src/channels/channels.controller.ts

@@ -24,6 +24,7 @@ export class ChannelsController {
         guildId: body.guildId as string | undefined,
         name: body.name as string | undefined,
         kind: body.kind as string | undefined,
+        xType: body.xType as string | undefined,
         isPublic: Boolean(body.isPublic),
         memberUserIds: Array.isArray(body.memberUserIds) ? (body.memberUserIds as string[]) : [],
       },