fix(deploy): prevent Caddy compression from buffering SSE

Replace the broad text response matcher with an explicit non-SSE MIME allowlist. Document proxy behavior and enforce the canonical Caddy compression policy in CI.

.github/workflows/backend-ci.yml

@@ -16,6 +16,7 @@ jobs:
         run: |
           /bin/bash -n deploy/apple-container.sh
           /bin/bash deploy/tests/apple-container-test.sh
+          /bin/sh deploy/test-caddyfile-cache.sh
 
   test:
     runs-on: ubuntu-latest