doc: notes about mountpoint hurdles

src/backend/src/modules/filesystem/roadmap.md

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+## Mountpounts hurdles
+
+- [ ] subdomains use integer IDs to to reference files, which
+      only works with PuterFS. This means other filesystem
+      providers will not be usable for subdomains.
+
+      Possible solutions:
+      - GUI logic to disable subdomains feature for other providers
+      - Add a new column to associate subdomains with paths
+      - Map non-puterfs nodes to (1B + path_id), where path_id is
+        a numeric identifier that is associated with the path, and
+        the association is stored in the database or system runtime
+        directory.
+
+- [ ] permissions are associated with UUIDs, but will need to
+      be able to be associated with paths instead for non-puterfs
+      mountpoints.
+
+      - Make path-to-uuid re-writer act on puter-fs only.
+      - ACL needs to be able to check path-based permissions
+        on non-puterfs mountpoints.