feat(infra): bind-mount /import for backend mass-import endpoint

`MassImportService` reads the ODS spreadsheet and referenced PDFs from a
hardcoded `/import` path inside the backend container. Dev compose
already bind-mounts `./import:/import`, but the prod compose had no
equivalent, so `POST /api/admin/import` would always fail on staging/prod
with "no spreadsheet found".

Mount strategy:
- Source path is env-driven (`IMPORT_HOST_DIR`), defaulting to
  `/srv/familienarchiv/import` so the host path is stable across CI
  deploys (the compose working dir is recreated each run, so `./import`
  would not persist).
- Read-only — `MassImportService` only reads (`Files.list` /
  `Files.walk`), never writes. Read-only mount makes that contract
  explicit and prevents the backend container from mutating the source
  PDFs.
- Empty / missing path is harmless: the import API just returns the
  existing "no spreadsheet found" error rather than crashing the
  container.

To use on staging: rsync the import folder to
`/srv/familienarchiv-staging/import/` on the host, set
`IMPORT_HOST_DIR=/srv/familienarchiv-staging/import` in `.env.staging`,
redeploy, trigger import from `/admin/system`.

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

docker-compose.prod.yml

@@ -26,6 +26,14 @@
 #   MAIL_HOST, MAIL_PORT,       SMTP relay (production only; staging uses mailpit)
 #   MAIL_USERNAME, MAIL_PASSWORD
 #   APP_MAIL_FROM               sender address (e.g. noreply@raddatz.cloud)
+#
+# Optional env vars:
+#   IMPORT_HOST_DIR             absolute host path holding the ODS spreadsheet
+#                               and PDFs for /admin/system mass import. Mounted
+#                               read-only at /import inside the backend.
+#                               Defaults to /srv/familienarchiv/import. When the
+#                               path is empty the import API simply reports
+#                               "no spreadsheet found" — no crash.
 
 networks:
   archiv-net:
@@ -173,6 +181,12 @@ services:
     # Bound to localhost only — Caddy fronts external traffic.
     ports:
       - "127.0.0.1:${PORT_BACKEND}:8080"
+    # Host path holding the ODS spreadsheet + PDFs for the mass-import endpoint.
+    # Read-only; MassImportService only reads (Files.list / Files.walk on /import).
+    # Outside the compose working dir on purpose — that dir is recreated per CI
+    # deploy. See IMPORT_HOST_DIR in the header for the env-var contract.
+    volumes:
+      - ${IMPORT_HOST_DIR:-/srv/familienarchiv/import}:/import:ro
     environment:
       SPRING_DATASOURCE_URL: jdbc:postgresql://db:5432/archiv
       SPRING_DATASOURCE_USERNAME: archiv