feat(gmw): public features #2-#6 — live moderation feed, toxic topic trends, channel timeline, CSV export, activity heatmap

- Live Moderation Feed: gateway publishes discord:moderation:action (Redis) → backend WS emits moderation_action → public web shows realtime stream.
- Toxic Topic Trends: backend moderation.trends aggregates categories/severity/action_type (read-only) → SVG bar + donut.
- Channel Timeline: messages view gets Feed/Timeline toggle with date-grouped separators.
- CSV Export: client-side downloadCsv for moderation actions (no backend write scope).
- Activity Heatmap: backend messages.activity (per-hour volume by channel) → pure-SVG grid.

User reputation deliberately excluded — no such feature exists in the codebase.
All read-only / public-facing / fully automatic per project rules.

services/frontend/src/hooks/use-messages.ts

@@ -5,6 +5,7 @@ import { messagesApi, voiceApi } from "@/lib/api";
 import type {
   AttachmentRecord,
   Channel,
+  MessageActivityBucket,
   MessageRecord,
   SemanticSearchResult,
 } from "@/lib/types";
@@ -374,3 +375,9 @@ export function useMessagesStream(
 
   return { streaming, error };
 }
+
+export function useMessageActivity(days = 30) {
+  return useSWR<MessageActivityBucket[]>(["activity", days], () =>
+    messagesApi.getActivity(days),
+  );
+}