fix: drop doubled felhom-data segment from HDD app compose templates

Model A binds the guest mount /mnt/<drive> directly onto the host's
<drive>/felhom-data namespace, so the guest mount already IS felhom-data.
The templates' ${HDD_PATH}/felhom-data/appdata/<app> therefore double-nested
to <drive>/felhom-data/felhom-data/appdata/<app> on disk, diverging from the
provenance-aware backup helpers (NamespaceRoot(drive,true) -> single-nested).

Change all four HDD app templates (romm, nextcloud, immich, paperless-ngx)
to ${HDD_PATH}/appdata/<app>, matching AppDataDir(NamespaceRoot(HDD_PATH,true)).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

templates/immich/docker-compose.yml

@@ -9,7 +9,7 @@
 #   DB_PASSWORD     - PostgreSQL password (auto-generated)
 #
 # Storage layout:
-#   User photos/videos → ${HDD_PATH}/felhom-data/appdata/immich  (HDD, host path)
+#   User photos/videos → ${HDD_PATH}/appdata/immich  (HDD, host path)
 #   PostgreSQL data    → immich_postgres_data          (named volume, NVMe)
 #   ML model cache     → immich_ml_cache               (named volume, NVMe)
 #   Redis data         → immich_redis_data             (named volume, NVMe)
@@ -36,7 +36,7 @@ services:
       - IMMICH_MACHINE_LEARNING_URL=http://immich-machine-learning:3003
       - TZ=Europe/Budapest
     volumes:
-      - ${HDD_PATH}/felhom-data/appdata/immich:/usr/src/app/upload
+      - ${HDD_PATH}/appdata/immich:/usr/src/app/upload
     networks:
       - traefik-public
       - immich-internal