[Enhancement]: Individual boards mode #128

Problem / motivation

Today a board is shared: one check per cell for the whole team (@@unique([cellId]) on CheckedCell). There is no competition between players.

Proposed solution

• New board mode: each player gets their own shuffle of the same phrase set.
• First player to reach bingo wins; others see the leaderboard live.
• Requires a schema change (per-player board or per-user checks) and a migration.

Alternatives considered

Keep a shared board but credit each check to a player and rank by number of checks.

Area

Boards / Play