chore(16-03): add eslint-plugin-security to root flat config (D-03)

- Install eslint-plugin-security@3.0.1 to root devDependencies (pinned exact version; 3.0.1 chosen over 4.0.1 for bake time per RESEARCH)
- Add pluginSecurity import to eslint.config.js
- Insert security config block (files: apps/**/*.{ts,tsx}) with configs.recommended spread, placed before prettierConfig which stays last
- ESLint version unchanged at 9.39.4 (D-13-ESLint-PIN)

package.json

@@ -21,6 +21,7 @@
     "eslint-config-prettier": "10.1.8",
     "eslint-plugin-react": "7.37.5",
     "eslint-plugin-react-hooks": "7.1.1",
+    "eslint-plugin-security": "3.0.1",
     "markdownlint-cli2": "0.22.1",
     "prettier": "3.8.4",
     "typescript-eslint": "8.61.0"