chore(lint): clean up stale .eslintrc.json, add @eslint/js

Remove legacy .eslintrc.json (flat config via eslint.config.js was
already active). Add @eslint/js as explicit devDependency.

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

Author: vveerrgg

.eslintrc.json

@@ -90,6 +90,7 @@
     "@types/cors": "^2.8.19",
     "@types/express": "^4.17.21",
     "@types/node": "^20.19.33",
+    "@eslint/js": "^9.27.0",
     "@typescript-eslint/eslint-plugin": "^8.56.0",
     "@typescript-eslint/parser": "^8.56.0",
     "@vitest/coverage-v8": "^3.2.4",