catalog: repoint HDD_PATH defaults/placeholders to /mnt/felhom-drives/<name>

Intermediary-mount re-architecture: drives are visible in-guest at the stable
/mnt/felhom-drives/<name>. Composes already use ${HDD_PATH}/${USERDATA_PATH}
(injected + repointed by controller v0.67.0); this updates the UI placeholders,
templates.json defaults, and doc/script examples to the new convention.

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

README.md

@@ -143,7 +143,7 @@ deploy_fields:
     label: "Adattárolási útvonal"
     type: path
     required: true
-    placeholder: "/mnt/hdd_1"
+    placeholder: "/mnt/felhom-drives/hdd_1"
     locked_after_deploy: true
 ```
 
@@ -290,7 +290,7 @@ where the felhom-controller is not used. For controller-based deployments, these
 ```bash
 # Generate customer templates (on your workstation)
 ./scripts/generate-customer.sh --customer demo-felhom \
-    --domain demo-felhom.eu --hdd-path /mnt/hdd_1 --push
+    --domain demo-felhom.eu --hdd-path /mnt/felhom-drives/hdd_1 --push
 ```
 
 ## Adding a New App