Tile position editor cannot reorder into an occupied position

[YoyoJesus]

Problem

The admin tiles page promises reordering by position, but bingo_tile.position is unique and the update action writes the submitted position directly. Moving a tile to any existing position causes PostgreSQL's unique-constraint error instead of changing the order; swapping two positions is impossible through the UI.

Reproduction

1. Open /admin/tiles with two tiles at positions 0 and 1.
2. Change the position of tile 0 to 1 and save.
3. The database rejects the update on the unique position constraint, currently surfaced as a server error.

Suggested fix

Implement a reorder operation that adjusts the intervening positions in a transaction (or use a temporary unused position for swaps), and return a validation error rather than an unhandled database error for conflicts.

Affected code

• src/lib/server/db/schema.ts
• src/routes/admin/tiles/+page.server.ts
• src/routes/admin/tiles/+page.svelte