ci(nightly): regression guard for backend /import:ro mount

Sara flagged that a future "compose cleanup" PR could silently drop the
backend volumes block and CI would happily pass while mass import on
staging silently broke. Adds a pre-build step that renders the staging
compose config and fails the deploy if `target: /import` or
`read_only: true` is missing.

Local verification of the guard:
- Volumes block removed → `grep -q 'target: /import'` exits 1 → step fails
- Volumes block present → both greps match → step passes

Addresses Sara's review on #526.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/nightly.yml

@@ -76,6 +76,28 @@ jobs:
           IMPORT_HOST_DIR=/srv/familienarchiv-staging/import
           EOF
 
+      - name: Verify backend /import:ro mount is wired
+        # Regression guard for #526: the /admin/system mass-import card
+        # only works when the backend service mounts the host import
+        # payload at /import (read-only). If a future "compose cleanup"
+        # PR drops the volumes block, mass import silently breaks again.
+        # `compose config` renders both shorthand and longform mounts as
+        # `target: /import` + `read_only: true`, so we assert against
+        # the rendered form rather than the raw source YAML.
+        run: |
+          set -e
+          docker compose \
+            -f docker-compose.prod.yml \
+            -p archiv-staging \
+            --env-file .env.staging \
+            --profile staging \
+            config > /tmp/compose-rendered.yml
+          grep -q '^[[:space:]]*target: /import$' /tmp/compose-rendered.yml \
+            || { echo "::error::backend is missing the /import bind mount (see #526)"; exit 1; }
+          grep -A2 '^[[:space:]]*target: /import$' /tmp/compose-rendered.yml \
+            | grep -q 'read_only: true' \
+            || { echo "::error::backend /import mount is not read-only (see #526)"; exit 1; }
+
       - name: Build images
         # `--pull` forces re-fetching pinned base images so a CVE
         # re-publication of the same tag (e.g. node:20.19.0-alpine3.21,