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/Header.tsx

@@ -10,13 +10,13 @@ import type { WsStatus } from "../shared/ws/socket";
 const titles: Record<DashboardTab, string> = {
   live: "Voice & Media",
   messages: "Messages & Moderation",
-  tuner: "Prompt Tuner",
+  dashboard: "Dashboard",
 };
 
 const subtitles: Record<DashboardTab, string> = {
   live: "Join voice channels, play media, stream audio, and browse recordings.",
   messages: "Capture, analyse, and moderate Discord messages.",
-  tuner: "Monitor correction patterns and improve AI moderation accuracy.",
+  dashboard: "Server statistics, user profiles, and AI moderation overview.",
 };
 
 interface HeaderProps {