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/common/api-key.guard.ts

@@ -4,7 +4,7 @@ import {
   Injectable,
   UnauthorizedException,
 } from '@nestjs/common';
-import { introspectGuildToken } from './center-auth';
+import { introspectGuildToken } from './center-auth.js';
 
 @Injectable()
 export class ApiKeyGuard implements CanActivate {

src/common/metrics.controller.ts

@@ -1,5 +1,5 @@
 import { Controller, Get } from '@nestjs/common';
-import { MetricsService } from './metrics.service';
+import { MetricsService } from './metrics.service.js';
 
 @Controller('metrics')
 export class MetricsController {

src/common/request-context.middleware.ts

@@ -1,6 +1,6 @@
 import { randomUUID } from 'crypto';
 import { NextFunction, Request, Response } from 'express';
-import { MetricsService } from './metrics.service';
+import { MetricsService } from './metrics.service.js';
 
 type ReqWithId = Request & { requestId?: string };