feat(dashboard): message activity timeline + moderation donut

Backend:
- GET /api/dashboard/activity?days=1..90 — daily buckets (messages,
  flagged, active_users) + hourly distribution last 24h
- clamped days param, reuses existing indexes (idx_messages_created,
  ai_status_created)

Frontend:
- ActivityChart: area chart messages+flagged per day, 7/14/30d range
- HourlyActivityChart: 24h bars with peak highlight
- ModerationDonut: clean/flagged/warned/error breakdown with live
  summary line (server X% clean)
- Dashboard layout: activity 2/3 + donut 1/3, hourly + top channels

Verified: endpoint returns real data from prod DB (784/1897/8 msgs
per day), tsc clean backend+frontend.

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

@@ -19,6 +19,16 @@ export function createDashboardRouter(): Router {
     }),
   );
 
+  // GET /api/dashboard/activity?days=14 — message volume over time
+  router.get(
+    "/dashboard/activity",
+    asyncHandler(async (req: Request, res: Response) => {
+      const days = Math.min(Math.max(Number(req.query.days) || 14, 1), 90);
+      const activity = await dashboardService.getActivity(days);
+      res.json(activity);
+    }),
+  );
+
   // GET /api/dashboard/users — paginated user list with profiles
   router.get(
     "/dashboard/users",