feat(dashboard): expose trust reputation + reactions leaderboard

- GET /api/dashboard/reactions: top reacted messages (net add-remove),
  joined with message content, channel name, top 3 emoji breakdown
- Users tab: colored trust tier badge (Trusted/Netral/At Risk/Kritis)
  in list rows + detail panel (was plain number badge)
- Dashboard: new Reactions sub-tab with leaderboard
  (rank, emoji cluster, message, author, channel, count)

services/backend/src/modules/dashboard/dashboard.service.ts

@@ -43,6 +43,11 @@ export class DashboardService {
     logger.debug({ channelId }, "Fetching channel detail");
     return dashboardRepository.getChannelDetail(channelId);
   }
+
+  async getTopReactions(limit: number) {
+    logger.debug({ limit }, "Fetching top reactions");
+    return dashboardRepository.getTopReactions(limit);
+  }
 }
 
 export const dashboardService = new DashboardService();