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/shared/api/client.ts

@@ -167,6 +167,17 @@ export function reanalyzeMessage(id: string): Promise<void> {
   return request<void>(`/api/messages/${id}/reanalyze`, { method: "POST" });
 }
 
+export function reanalyzeErrorBatch(opts: {
+  guildId?: string;
+  channelId?: string;
+  messageIds?: string[];
+}): Promise<{ ok: boolean; count: number }> {
+  return request<{ ok: boolean; count: number }>(
+    "/api/messages/reanalyze-batch",
+    { method: "POST", body: JSON.stringify(opts) },
+  );
+}
+
 export function moderateMessage(
   id: string,
   actionType: string,