refactor: migrate to ES modules

package.json type=module, tsconfig module/moduleResolution=NodeNext,
target es2022, explicit .js on all relative imports. Center: jsonwebtoken
& bcryptjs switched to default imports (ESM/CJS interop). Verified:
builds, boots, full auth + plugin round-trip work under ESM.

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

src/channels/channels.controller.ts

@@ -1,5 +1,5 @@
 import { Body, Controller, Get, Param, Post, Query, Req, UnauthorizedException } from '@nestjs/common';
-import { ChannelsService } from './channels.service';
+import { ChannelsService } from './channels.service.js';
 
 // ApiKeyGuard attaches the introspected Center user id onto the request.
 type AuthedRequest = { userId?: string };