Title: fix(reputation): remove mock leaderboard path, add insufficient-data response

[chidinma000]

src/api/reputation.ts previously had two leaderboard code paths — one that built a synthetic mockChainData array (uniform transactionCount: 10, a sybilRisk derived from combinedScore) and passed it to createLeaderboard. A reputation leaderboard that can silently return fabricated rankings is misleading, and the dual path was a maintenance hazard. The synthetic path was already deleted on main in 3992a6c

Closes #47