ci: validate both Caddyfiles in the test job (#26)

The test image swaps in Caddyfile.test, so the production Caddyfile was
never exercised by CI: the parse error from issue #20 sailed through
every check and only surfaced when the deploy gate failed — after the
broken container had already replaced the healthy one. Running
`caddy validate` (same caddy:2-alpine base as the Dockerfile) on both
files up front fails the PR check in seconds instead.

Touches the frozen .github/workflows/ per explicit product-owner
approval in #26.

Closes #26

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/test.yml

@@ -11,6 +11,18 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      # The test image below swaps in Caddyfile.test, so the production
+      # Caddyfile is otherwise never exercised by CI — an invalid config then
+      # only surfaces when the deploy health-gate fails, after the broken
+      # container already replaced the healthy one (issue #20/#26). Validate
+      # both files up front so a parse error fails the PR check in seconds.
+      - name: Validate Caddyfiles
+        run: |
+          docker run --rm -v "$PWD":/workspace -w /workspace caddy:2-alpine \
+            caddy validate --config Caddyfile --adapter caddyfile
+          docker run --rm -v "$PWD":/workspace -w /workspace caddy:2-alpine \
+            caddy validate --config Caddyfile.test --adapter caddyfile
+
       - name: Build Docker image
         run: docker build -t learning-platform .