feat: reorder child entries with a configurable role permission

- Persist sibling order for child entries (archived_entries.position):
  backfilled to the previous archived_at order, new children appended
  (initial playlist order kept; sync appends after user order).
- PUT /api/archives/:id/entries/:uid/children/order takes the full child
  UID list in one IMMEDIATE transaction (401 guest, 403 role not allowed,
  404 unknown or invisible parent, 400 unless exactly the current children).
- Reorder permission is a role mask in the auth instance settings
  (reorder_children_role_bits, default Admin|Owner). Only the Owner can
  change it, built-in or custom roles, never Guest. /api/auth/me and login
  expose can_reorder_children. Settings PATCH is now one IMMEDIATE txn.
- Main page: drag handle for mouse/trackpad on viewports >640px, up/down
  arrows otherwise (pure CSS, arrows are the fallback), Alt+Up/Down
  everywhere; optimistic with revert. Settings > Instance > Permissions
  for the Owner (read-only for admins).
- Fix: renaming a child entry now updates its row immediately.
- Remove Storybook (deps, config, stories, docs).

.gitignore

@@ -42,5 +42,3 @@ frontend/result/**
 frontend/dist/
 frontend/dist/**
 .DS_Store
-frontend/storybook-static/
-frontend/storybook-static/**