feat: replace corrections/tuner with dashboard module

Replace the corrections/adaptive-prompt-tuner feature with a new
dashboard module providing server stats and user profile overview.

Backend:
- Add dashboard module (routes, service, repository) with stats + user list + user detail endpoints
- Remove corrections module entirely
- Wire dashboard router in app.ts

Frontend:
- Add dashboard feature (DashboardStats, UserSummaryList, UserProfileDetail components + useDashboard hook)
- Remove tuner feature (CorrectionStats, CorrectionHistory, SubmitCorrection, useCorrections)
- Update API client from corrections → dashboard types/fns
- Rename tab 'tuner' → 'dashboard'
- Update MobileTabBar, Header, Sidebar links

Tests:
- Expand backend placeholder test with dashboard assertions
- Expand discord-gateway placeholder test with config/channel assertions

AI moderation:
- llmModerationClient: improve status/reply detection, expand safety categories, fix timer reset
- userProfileLearner: fix isReply refinement
- userProfileStore: add pending cache check
- messageMetadata: add crosspost type mapping
- migrate.ts: improve partial-index safety in schema push

Co-Authored-By: Claude <noreply@anthropic.com>

services/frontend/src/widgets/Sidebar.tsx

@@ -1,5 +1,5 @@
 import { motion } from "framer-motion";
-import { MessageSquare, Radio, SlidersHorizontal } from "lucide-react";
+import { LayoutDashboard, MessageSquare, Radio } from "lucide-react";
 import type { DashboardTab } from "../entities/ui/types";
 import type { MessageRecord } from "../shared/api/client";
 import { useMascotChat } from "../shared/hooks/useMascotChat";
@@ -11,7 +11,7 @@ const navItems: Array<{ id: DashboardTab; label: string; icon: typeof Radio }> =
   [
     { id: "live", label: "Live", icon: Radio },
     { id: "messages", label: "Messages", icon: MessageSquare },
-    { id: "tuner", label: "Tuner", icon: SlidersHorizontal },
+    { id: "dashboard", label: "Dashboard", icon: LayoutDashboard },
   ];
 
 interface SidebarProps {