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/discord-gateway/src/app/bootstrap.ts

@@ -25,6 +25,7 @@ import {
   registerMessageCapture,
   setEventBroadcaster as setMessageCaptureEventBroadcaster,
 } from "../modules/message-capture/messageCapture.js";
+import { setModerationEventBroadcaster } from "../modules/message-capture/moderationActionsDb.js";
 import { registerReactionCapture } from "../modules/reaction-tracking/index.js";
 import { registerThreadCapture } from "../modules/thread-tracking/index.js";
 import { registerPresenceCapture } from "../modules/user-presence/index.js";
@@ -254,6 +255,7 @@ export async function initializeDiscordGateway() {
     logger.info({ user: client.user?.tag }, "Bot logged in");
     setMessageCaptureEventBroadcaster(eventBroadcaster);
     setRecorderEventBroadcaster(eventBroadcaster);
+    setModerationEventBroadcaster(eventBroadcaster);
     registerMessageCapture(client);
     startPendingAIAnalysisWorker(client, eventBroadcaster);