Optimize reorderLists with Supabase RPC bulk update

[gdiab]

Problem

reorderLists in ListsContext.tsx currently issues N sequential Supabase .update() calls — one per active list — when the user reorders via drag-and-drop or move up/down. This works but is inefficient for users with many lists.

Proposed Solution

Create a Supabase RPC (PostgreSQL function) that accepts an array of {id, sort_order} pairs and performs a single bulk UPDATE statement. Call this RPC from reorderLists instead of looping.

Acceptance Criteria

• Create a new migration with a reorder_lists(updates jsonb) RPC function
• Update reorderLists in ListsContext.tsx to call the RPC
• Verify optimistic UI update still works
• No regressions in custom sort behavior