fix(leaderboard): evict deleted users from cached leaderboard results

[mallya-m]

What does this PR do?

Fixes deleted users appearing in the leaderboard indefinitely. Cached results are now filtered against current public users in Supabase before serving.

Related issue

Closes #934

Changes made

• Added filterDeletedUsers() that cross-references cached usernames against current public users in DB
• Applied to both memory cache and distributed cache hits
• Re-ranks remaining entries after filtering
• Also fixes pre-existing build error: missing bio field in public-profile-data.ts

How to test

1. Have a user on the leaderboard
2. Set their is_public = false in Supabase
3. Leaderboard no longer shows them without server restart

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

Closing — superseded by PR #1616 which includes the same leaderboard changes plus additional improvements to GoalTracker and other components. Please follow the discussion on #1616.