fix: add backend prettier config without tailwindcss plugin

- Create separate .prettierrc for backend to avoid inheriting frontend's prettier-plugin-tailwindcss
- This fixes the CI linting error where the plugin was not found

Author: Your Name

backend/.prettierrc

@@ -0,0 +1,13 @@
+{
+  "semi": true,
+  "trailingComma": "es5",
+  "singleQuote": true,
+  "printWidth": 100,
+  "tabWidth": 2,
+  "useTabs": false,
+  "quoteProps": "as-needed",
+  "bracketSpacing": true,
+  "bracketSameLine": false,
+  "arrowParens": "always",
+  "endOfLine": "lf"
+}