[Bug]: Separate public leaderboard cache from user-specific rank data

[cyclofan]

Problem description

The leaderboard API mixes public leaderboard data with optional user-specific rank data.

Public leaderboard responses are good candidates for CDN caching, but user-specific data should be handled separately to avoid cache confusion and make privacy boundaries clearer.

Expected behavior

The project should have a clearer separation between:

• Public anonymous leaderboard data
• User-specific rank/profile data

Reproduction steps

Review the current leaderboard API behavior and caching strategy.

Initial files to inspect:

• app/api/leaderboard/all/route.ts
• app/api/leaderboard/global/route.ts
• app/api/leaderboard/profile/route.ts
• lib/kv.ts

Impact

• Safer caching model
• Clearer privacy boundary
• Better API maintainability
• Easier future public API design

Suggested direction

• Keep public leaderboard endpoints cache-friendly.
• Move user-specific rank/profile reads to separate endpoints.
• Document the intended API boundary.
• Avoid changing ranking behavior in the first PR unless necessary.