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/backend/src/modules/messages/messages.service.ts

@@ -101,6 +101,10 @@ export class MessagesService {
     const results = hits.map((h) => mapSearchHit(h));
     return { results, nextCursor: null };
   }
+
+  async getActivity(days = 30) {
+    return messagesRepository.getActivity(days);
+  }
 }
 
 /** Shape returned to the frontend (text + metadata from the archive payload). */