Add model usage leaderboard

[Chel-Tbl]

Summary

• add a public model usage aggregation query over assistant messages
• add a /models leaderboard page with model counts, provider, context length, capabilities, and usage share
• link the leaderboard from the logged-out navbar and authenticated sidebar

Verification

• tsc --noEmit --pretty false
• targeted prettier --check for changed files
• SKIP_ENV_VALIDATION=1 next build

Note: Browser verification was blocked locally because this workspace has no .env.local and Docker is not installed in PATH, so the database-backed page cannot be served with real data here.

Closes #220