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>

package.json

@@ -2,6 +2,7 @@
   "name": "fabric-backend-guild",
   "version": "0.1.0",
   "private": true,
+  "type": "module",
   "description": "Fabric Guild Node service",
   "scripts": {
     "build": "tsc -p tsconfig.build.json",