Leaderboard sort among flagged/unranked rows is unstable

**Severity:** Low `src/server/leaderboard.ts:83` sorts with `(a.rank ?? Infinity) - (b.rank ?? Infinity)`, mapping every flagged/unranked row to `Infinity`, so their relative order is implementation-defined. Cosmetic (unranked rows have no rank to preserve). **Fix** Add a deterministic tiebreak for equal/absent ranks (e.g. by `createdAt` or `display` name). **Acceptance:** flagged rows render in a stable, deterministic order.