Persistent leaderboard: all-time rankings per cup

Description

After the alpha proves the core loop works, players want to see how they stack up historically. An all-time leaderboard per cup (top 10 scores) gives players a reason to replay and compete long-term.

Acceptance Criteria

• LeaderboardEntry entity stores: cup name, player UUID, score, position, date
• After each race ends: leaderboard is updated if the player's score is a personal best for that cup
• /leaderboard <cup-name> command displays the top 10 all-time scores in chat
• Leaderboard query is cached in memory (TTL: 60 seconds) to avoid repeated DB reads
• Cache is invalidated when a new entry would enter the top 10
• Unit test covers: submit score → check leaderboard updated; submit lower score → check no update

Technical Details

• New LeaderboardEntity schema in shared/database
• LeaderboardService interface + impl in server module
• /leaderboard command uses the Cloud command framework

Dependencies

• "Persist game results in EndPhase" must be done first

Estimate

M

Start by checking the existing EndPhase game-result persistence and the shared/database module, then inspect server services and the Cloud command framework. Add the LeaderboardEntity and LeaderboardService flow described in the acceptance criteria, including the in-memory TTL cache. Done means the /leaderboard command shows the top 10 and unit tests cover both a personal-best update and a lower-score no-op.