feat: Add team/club leaderboards for inter-department and inter-hostel competitions

[amaydixit11]

Feature Request### ProblemWhile Organization and OrganizationMember models exist (models.py lines 234-270) with a /api/organizations/<pk>/leaderboard/ endpoint, there's no public-facing team leaderboard system.### Proposed Solution1. A /api/teams/ endpoint showing ranked teams by average unified score2. Filter by organization, department, or custom team tags3. Real-time team score updates during contests4. Frontend team leaderboard page with avatars and progress bars### Technical NotesThe existing OrganizationViewSet.leaderboard() action (views.py lines 1000-1038) already queries platform data for organization members. This could be extended to aggregate and rank teams rather than just list member scores.### Labelsenhancement, frontend, backend