fix(mounts): scope external mounts to drives

migrations/20261025000000_external_mount_drive_visibility.sql

@@ -0,0 +1,12 @@
+-- External mounts inherit access from the drive containing their mount-root
+-- folder. Existing mounts remain in their current drive; for the historical
+-- default that is the configuring administrator's personal drive.
+UPDATE storage.external_mounts
+SET visibility = 'drive'
+WHERE visibility = 'owner';
+
+ALTER TABLE storage.external_mounts
+    ALTER COLUMN visibility SET DEFAULT 'drive';
+
+COMMENT ON COLUMN storage.external_mounts.visibility IS
+    'Compatibility marker. External mount visibility is inherited from the mount-root folder drive and its role grants.';