feat(leaderboard): limit ranking queries to top 20

[KurosawaGeeker]

Summary

• Limit leaderboard page, API, and leaderboard JSON-LD queries to the shared top-20 leaderboard cap.
• Remove client-side pagination so leaderboard views cannot enumerate entries beyond the capped result set.
• Add 100-entry development preview data while only returning the requested capped subset to the page.
• Stabilize profile comment ordering when comments are created within the same millisecond.

Validation

• pnpm typecheck
• pnpm test (14 files / 128 tests passed)
• pnpm lint
• curl http://localhost:3001/leaderboard?view=heat only includes mock-dev-001 through mock-dev-020.

Generated Files

No generated files, GraphQL codegen, or DB baseline updates included.

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