perf(ci): combine test and build into single job

Eliminates duplicate job overhead:
- No separate job startup time
- No duplicate code checkout
- No runner scheduling delay
- Runs sequentially in same environment

Docker build still compiles from scratch (Docker isolation) but saves ~2-3min
in job overhead. Next step: optimize Dockerfile for better layer caching.

.forgejo/workflows/build.yaml

@@ -15,7 +15,7 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  test:
+  test-and-build:
     runs-on: ubuntu-22.04
     env:
       DATABASE_URL: postgresql://postgres:postgres@postgres:5432/towerops_test
@@ -73,13 +73,6 @@ jobs:
           mix compile --warnings-as-errors
           mix test --warnings-as-errors --max-cases 3
 
-  build:
-    runs-on: ubuntu-22.04
-    needs: test
-    steps:
-      - name: Checkout code
-        uses: actions/checkout@v4
-
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
         with: