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/orpc/router.ts

@@ -143,6 +143,14 @@ const messagesRouter = {
   semanticSearch: os
     .input(semanticSearchSchema)
     .handler(({ input }) => messagesService.semanticSearch(input)),
+  // Public, read-only activity heatmap data (per-hour volume by channel).
+  activity: os
+    .input(
+      z.object({
+        days: z.coerce.number().int().positive().max(365).default(30),
+      }),
+    )
+    .handler(({ input }) => messagesService.getActivity(input.days)),
 };
 
 // ── Moderation ───────────────────────────────────────────────────
@@ -165,6 +173,13 @@ const moderationRouter = {
         cursor: input.cursor,
       }),
     ),
+  trends: os
+    .input(
+      z.object({
+        days: z.coerce.number().int().positive().max(365).default(30),
+      }),
+    )
+    .handler(({ input }) => moderationService.getTrends(input.days)),
 };
 
 // ── Media ────────────────────────────────────────────────────────