ci(workflows): enhance workflows with matrix builds and caching optimizations

Refactor GitHub Actions workflows to introduce matrix builds for `backend`, `frontend`, and `documentation` components. Upgrade actions versions, add pull request triggers, and improve caching with pnpm store integration. Adjust Node.js version to 20 and enforce `--frozen-lockfile` for dependency installation.

.gitea/workflows/backend-tests.yml

@@ -1,8 +1,12 @@
 name: Backend Tests
+
 on:
   push:
     paths:
       - 'backend/**'
+  pull_request:
+    paths:
+      - 'backend/**'
 
 jobs:
   test:
@@ -14,9 +18,19 @@ jobs:
           version: 9
       - uses: actions/setup-node@v4
         with:
-          node-version: 22
-          cache: 'pnpm'
+          node-version: 20
+      - name: Get pnpm store directory
+        id: pnpm-cache
+        shell: bash
+        run: |
+          echo "STORE_PATH=$(pnpm store path --silent)" >> "${GITEA_OUTPUT:-$GITHUB_OUTPUT}"
+      - uses: actions/cache@v4
+        with:
+          path: ${{ steps.pnpm-cache.outputs.STORE_PATH }}
+          key: ${{ runner.os }}-pnpm-store-${{ hashFiles('**/pnpm-lock.yaml') }}
+          restore-keys: |
+            ${{ runner.os }}-pnpm-store-
       - name: Install dependencies
-        run: pnpm install
+        run: pnpm install --frozen-lockfile
       - name: Run Backend Tests
         run: pnpm -F @memegoat/backend test