feat: tiered category leader awards with top 3, escalating points, and revoke

- Award top 3 placements per category (1st/2nd/3rd) instead of just #1
- Escalating points by week (week × [10, 5, 3] multipliers)
- Cumulative awards support (weekNumber=0)
- Revoke mutation to undo awards and reverse points
- Shared placement point calculations in lib/categoryLeaderPoints.ts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: prazgaitis