Add traverse access level for folder navigation without file access

When a permission is set on a nested folder, traverse is automatically
granted on all ancestor folders so users can navigate to it. Traverse
only shows subfolders in listings — files are hidden. This prevents
orphaned permissions where a user has access to a nested folder but
can't reach it.

Hierarchy: traverse < view < edit < admin

packages/backend/src/db/migrations/0024_add_traverse_access_level.sql

@@ -0,0 +1,3 @@
+-- Add 'traverse' access level to storage_folder_access enum
+-- traverse: allows folder to appear in listings but does not grant file access
+ALTER TYPE storage_folder_access ADD VALUE IF NOT EXISTS 'traverse' BEFORE 'view';

packages/backend/src/db/migrations/meta/_journal.json

@@ -169,6 +169,13 @@
       "when": 1774830000000,
       "tag": "0023_store_settings",
       "breakpoints": true
+    },
+    {
+      "idx": 24,
+      "version": "7",
+      "when": 1774840000000,
+      "tag": "0024_add_traverse_access_level",
+      "breakpoints": true
     }
   ]
 }