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/moderation/moderation.service.ts

@@ -8,10 +8,13 @@ const logger = createChildLogger("moderation.service");
 
 export class ModerationService {
   async getStats() {
-    logger.debug("Fetching moderation stats");
     return moderationRepository.getStats();
   }
 
+  async getTrends(days = 30) {
+    return moderationRepository.getTrends(days);
+  }
+
   async listActions(query: ListModerationQuery) {
     logger.debug({ query }, "Listing moderation actions");
     return moderationRepository.listActions(query);