ci: optimize GitHub Actions workflow (FRO-62)

- Add concurrency to cancel outdated runs on same branch
- Add NuGet package cache for faster .NET builds
- Replace skip jobs with single CI Success job
- Simplify workflow from 5 to 4 jobs

Author: pierrick-fonquerne

.github/workflows/ci.yml

@@ -6,6 +6,10 @@ on:
   pull_request:
     branches: [main, develop]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   changes:
     name: Detect changes
@@ -16,8 +20,6 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-        with:
-          fetch-depth: 0
 
       - name: Detect file changes
         uses: dorny/paths-filter@v3
@@ -49,6 +51,13 @@ jobs:
         with:
           dotnet-version: '8.0.x'
 
+      - name: Cache NuGet packages
+        uses: actions/cache@v4
+        with:
+          path: ~/.nuget/packages
+          key: ${{ runner.os }}-nuget-${{ hashFiles('src/backend/**/*.csproj') }}
+          restore-keys: ${{ runner.os }}-nuget-
+
       - name: Restore dependencies
         run: dotnet restore
 
@@ -58,15 +67,6 @@ jobs:
       - name: Test
         run: dotnet test --no-build --configuration Release --verbosity normal --filter "Category!=Email"
 
-  backend-skip:
-    name: Backend (.NET) (pull_request)
-    needs: changes
-    if: ${{ needs.changes.outputs.backend == 'false' }}
-    runs-on: ubuntu-latest
-    steps:
-      - name: Skip
-        run: echo "No backend changes detected - skipping build"
-
   frontend:
     name: Frontend (React)
     needs: changes
@@ -102,11 +102,16 @@ jobs:
       - name: Test
         run: npm run test -- --passWithNoTests
 
-  frontend-skip:
-    name: Frontend (React) (pull_request)
-    needs: changes
-    if: ${{ needs.changes.outputs.frontend == 'false' }}
+  ci-success:
+    name: CI Success
+    needs: [changes, backend, frontend]
+    if: always()
     runs-on: ubuntu-latest
     steps:
-      - name: Skip
-        run: echo "No frontend changes detected - skipping build"
+      - name: Check results
+        run: |
+          if [[ "${{ needs.backend.result }}" == "failure" || "${{ needs.frontend.result }}" == "failure" ]]; then
+            echo "One or more jobs failed"
+            exit 1
+          fi
+          echo "CI passed successfully"