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>

templates/immich/.felhom.yml

@@ -47,7 +47,7 @@ deploy_fields:
     label: "Adattárolási útvonal"
     type: path
     required: true
-    placeholder: "/mnt/hdd_1"
+    placeholder: "/mnt/felhom-drives/hdd_1"
     description: "A külső merevlemez elérési útja, ahol a fotók és videók tárolódnak"
     locked_after_deploy: true