fix(ci): update lockfile paths and switch backend to pnpm

Author: Eniola3321

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
         with:
           node-version: '20'
           cache: 'npm'
-          cache-dependency-path: frontend/package-lock.json
+          cache-dependency-path: package-lock.json
 
       - name: Install dependencies
         run: npm ci
@@ -57,15 +57,20 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
 
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
           node-version: '20'
-          cache: 'npm'
-          cache-dependency-path: backend/package-lock.json
+          cache: 'pnpm'
+          cache-dependency-path: backend/pnpm-lock.yaml
 
       - name: Install dependencies
-        run: npm ci
+        run: pnpm install --frozen-lockfile
         working-directory: backend
 
       - name: Generate Prisma Client