fix: downgrade nanoid to v3 for CommonJS compatibility

nanoid v5 is ESM-only and cannot be require()'d in a CommonJS module.
v3 is the last version with native CJS support.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/backend/package.json

@@ -28,7 +28,7 @@
     "graphql-ws": "^6.0.4",
     "graphql-yoga": "^5.13.4",
     "ioredis": "^5.6.1",
-    "nanoid": "^5.1.5",
+    "nanoid": "^3.3.11",
     "nodemailer": "^7.0.3",
     "pg": "^8.16.0",
     "slugify": "^1.6.6",