Update metrics dashboard filters

[JoaquinBN]

Summary

• Bucket validator growth by graduation date, with leaderboard graduation data and validator auto-award fallback.
• Remove the rejected curve from submission state trends while keeping rejected outcome counts in the bar chart.
• Add a filterable Community contributions table and date filtering support on the contributions API.

Validation

• npm run build
• env/bin/python -m py_compile api/metrics_views.py contributions/views.py api/tests.py
• Django test blocked by existing test database/migration seed data issue in contributions/migrations/0037_seed_featured_content.py.