ci(nightly): wire IMPORT_HOST_DIR=/srv/familienarchiv-staging/import

The compose file now requires IMPORT_HOST_DIR or refuses to start
(#526). Without this line the next nightly deploy would fail with a
clear interpolation error, but it should not fail — the staging
import payload already lives at this host path (rsync'd in #526).

Addresses Tobias's review on #526.

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

.gitea/workflows/nightly.yml

@@ -73,6 +73,7 @@ jobs:
           MAIL_SMTP_AUTH=false
           MAIL_STARTTLS_ENABLE=false
           APP_MAIL_FROM=noreply@staging.raddatz.cloud
+          IMPORT_HOST_DIR=/srv/familienarchiv-staging/import
           EOF
 
       - name: Build images