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.routes.ts

@@ -87,5 +87,15 @@ export function createDashboardRouter(): Router {
     }),
   );
 
+  // GET /api/dashboard/reactions — top reacted messages
+  router.get(
+    "/dashboard/reactions",
+    asyncHandler(async (req: Request, res: Response) => {
+      const limit = Number(req.query.limit) || 20;
+      const reactions = await dashboardService.getTopReactions(limit);
+      res.json(reactions);
+    }),
+  );
+
   return router;
 }