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.service.ts

@@ -15,6 +15,11 @@ export class DashboardService {
     return dashboardRepository.getStats();
   }
 
+  async getActivity(days: number) {
+    logger.debug({ days }, "Fetching dashboard activity");
+    return dashboardRepository.getActivity(days);
+  }
+
   async listUsers(query: ListUsersQuery) {
     logger.debug({ query }, "Listing dashboard users");
     return dashboardRepository.listUsers(query);