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/romm/docker-compose.yml

@@ -11,8 +11,8 @@
 #   ROMM_AUTH_SECRET_KEY    - Auth secret (auto-generated)
 #
 # Storage layout:
-#   ROM library    → ${HDD_PATH}/felhom-data/appdata/romm/library     (HDD, host path)
-#   Cover art etc  → ${HDD_PATH}/felhom-data/appdata/romm/resources   (HDD, host path)
+#   ROM library    → ${HDD_PATH}/appdata/romm/library     (HDD, host path)
+#   Cover art etc  → ${HDD_PATH}/appdata/romm/resources   (HDD, host path)
 #   App config     → romm_config                            (named volume, NVMe)
 #   MariaDB data   → romm_db_data                           (named volume, NVMe)
 #   Redis data     → romm_redis_data                        (named volume, NVMe)
@@ -62,8 +62,8 @@ services:
       - MOBYGAMES_API_KEY=${MOBYGAMES_API_KEY:-}
       - TZ=Europe/Budapest
     volumes:
-      - ${HDD_PATH}/felhom-data/appdata/romm/library:/romm/library
-      - ${HDD_PATH}/felhom-data/appdata/romm/resources:/romm/resources
+      - ${HDD_PATH}/appdata/romm/library:/romm/library
+      - ${HDD_PATH}/appdata/romm/resources:/romm/resources
       - romm_config:/romm/config
     networks:
       - traefik-public