fix(mounts): scope external mounts to drives

docs/external-mounts.md

@@ -0,0 +1,23 @@
+# External mounts
+
+External mounts expose files from a provider such as a host directory inside an
+OxiCloud drive. Enable the feature with `OXICLOUD_ENABLE_EXTERNAL_MOUNTS=true`,
+then configure mounts from **Administration > External Mounts**.
+
+Each mount must be attached to a drive. The mount-root is a normal folder in
+that drive, and access to all provider content is inherited from the drive's
+membership and roles:
+
+- a mount in a personal drive is visible only to that drive's owner;
+- a mount in a shared drive is visible to members who can read the drive;
+- mutations additionally require the corresponding drive permission and are
+  always rejected when the mount is configured as read-only.
+
+Removing a mount deletes only its OxiCloud mount-root and configuration. It does
+not delete the provider's root directory. Deleting files or folders *inside* a
+writable mount is permanent because external mounts do not use OxiCloud trash.
+
+Mounts created before drive selection was introduced remain attached to their
+existing drive, normally the configuring administrator's personal drive. To
+make one available through a shared drive, remove it and recreate it with that
+shared drive selected; removing the old mount does not remove host content.