CTF: Implement Leaderboard (rankings + levels + profile links)

[nitinawari]

Future Extension Ideas

Description
Add a Leaderboard page to the CTF portal that ranks players by points, shows their level progression, and links to public profiles. The leaderboard surfaces existing player data — points, solves, and badges — with a deterministic ranking algorithm.

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Features / Scope

Leaderboard Page

• Route: /ctf/leaderboard
• Columns: Rank · Player · Points · Level · Solves · Badges
• Username links to existing public profile at /ctf/h/{username}
• Shows empty state when no players exist

Leaderboard API

• Endpoint: GET /ctf/api/v1/leaderboard?limit=50
• Returns: rank, username, points, level, solves_count, badges_count, last_solve_at

Ranking Rules

• Primary: points DESC
• Tie-break 1: last_solve_at ASC (earlier achiever ranks higher)
• Tie-break 2: username ASC (final deterministic tie-break)

Level Progression

• Level is computed using the existing calculate_level(points) function as the single source of truth

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Why this matters:

• Improves engagement and competition
• Makes progress visible across users
• Uses already available data (no major infra change)

This was previously discussed and deferred — adding here for future backlog tracking.
issue #217