chore: remove auth rate limiting

Vector is an internal service — throttling /api/auth requests provides
no meaningful protection and gets in the way of legitimate use. Drops
the express-rate-limit middleware and dependency.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/api/package.json

@@ -22,7 +22,6 @@
     "cors": "^2.8.6",
     "dotenv": "^17.4.2",
     "express": "^5.2.1",
-    "express-rate-limit": "^8.3.2",
     "helmet": "^8.1.0",
     "jsonwebtoken": "^9.0.3",
     "pino": "^10.3.1",