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/frontend/src/lib/api/dashboard.ts

@@ -1,4 +1,5 @@
 import type {
+  DashboardActivity,
   DashboardChannelDetail,
   DashboardStats,
   DashboardUserDetail,
@@ -10,6 +11,9 @@ import { api } from "./client";
 export const dashboardApi = {
   getStats: () => api.get<DashboardStats>("/api/dashboard/stats"),
 
+  getActivity: (days = 14) =>
+    api.get<DashboardActivity>(`/api/dashboard/activity?days=${days}`),
+
   listUsers: (limit?: number, cursor?: string, search?: string) => {
     const params = new URLSearchParams();
     if (limit) params.set("limit", String(limit));

services/frontend/src/lib/types/dashboard.ts

@@ -28,6 +28,25 @@ export interface ModerationOverview {
   error: number;
 }
 
+export interface DashboardActivity {
+  days: number;
+  daily: DailyActivityPoint[];
+  hourly: HourlyActivityPoint[];
+}
+
+export interface DailyActivityPoint {
+  day: string; // YYYY-MM-DD
+  messages: number;
+  flagged: number;
+  active_users: number;
+}
+
+export interface HourlyActivityPoint {
+  hour: number; // 0-23
+  messages: number;
+  flagged: number;
+}
+
 export interface DashboardUser {
   user_id: string;
   username?: string | null;