feat: add "Retry All Errors" batch reanalyze button

Backend:
- POST /api/messages/reanalyze-batch — bulk reset error messages to pending
- messages.repository.reanalyzeErrorBatch() — scoped by guildId/channelId/messageIds
- messages.service.reanalyzeErrorBatch() — validation layer

Frontend:
- reanalyzeErrorBatch() API client function
- useMessages.reanalyzeAllErrors() — optimistic state + batch call
- MessagesPanel: destructive button visible when error count > 0
- Shows confirmation text with queued count

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/frontend/src/App.tsx

@@ -207,6 +207,7 @@ export default function App() {
           guildName={monitorGuildName}
           messages={messages.messages}
           onReanalyze={messages.reanalyze}
+          onReanalyzeAllErrors={messages.reanalyzeAllErrors}
           onLoadMore={messages.loadMore}
           hasMore={messages.hasMore}
           loadingMore={messages.loadingMore}