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.repository.ts

@@ -82,6 +82,52 @@ export class DashboardRepository {
     };
   }
 
+  async getActivity(days: number) {
+    const db = getDatabase();
+    const sinceMs = Date.now() - days * 86400000;
+    const dayAgoMs = Date.now() - 86400000;
+
+    // Daily buckets (last N days)
+    const daily = await db.execute(sql`
+      SELECT
+        to_char(to_timestamp(created_at / 1000), 'YYYY-MM-DD') AS day,
+        COUNT(*)::int AS messages,
+        COUNT(*) FILTER (WHERE ai_status = 'flagged')::int AS flagged,
+        COUNT(DISTINCT user_id)::int AS active_users
+      FROM ${pgMessagesTable}
+      WHERE created_at >= ${sinceMs}
+      GROUP BY day
+      ORDER BY day
+    `);
+
+    // Hourly distribution (last 24h)
+    const hourly = await db.execute(sql`
+      SELECT
+        EXTRACT(HOUR FROM to_timestamp(created_at / 1000))::int AS hour,
+        COUNT(*)::int AS messages,
+        COUNT(*) FILTER (WHERE ai_status = 'flagged')::int AS flagged
+      FROM ${pgMessagesTable}
+      WHERE created_at >= ${dayAgoMs}
+      GROUP BY hour
+      ORDER BY hour
+    `);
+
+    return {
+      days,
+      daily: (daily.rows as Record<string, unknown>[]).map((r) => ({
+        day: String(r.day),
+        messages: Number(r.messages),
+        flagged: Number(r.flagged),
+        active_users: Number(r.active_users),
+      })),
+      hourly: (hourly.rows as Record<string, unknown>[]).map((r) => ({
+        hour: Number(r.hour),
+        messages: Number(r.messages),
+        flagged: Number(r.flagged),
+      })),
+    };
+  }
+
   async listUsers(query: ListUsersQuery) {
     const db = getDatabase();
     const limit = query.limit ?? 20;