Writer leaderboard based on reactions and bookmarks #118
Description
Add a public leaderboard showing top writers ranked by total reactions and bookmarks received, filterable by time period (all time, this month).
Context: Gamification drives contribution — writers compete to get to the top, which creates more quality content.
Acceptance criteria:
- Ranked list with avatar, name, score
- Filter by: all time / this month / this week
- Link to writer profile