fix: type cast messages for DeepSeekProvider compatibility

src/modules/admin-ai-chat/admin-ai-chat.service.ts

@@ -63,7 +63,7 @@ export class AdminAiChatService {
     const start = Date.now();
     const result = await this.deepseek.generate({
       model: 'deepseek-chat',
-      messages,
+      messages: messages as Array<{ role: 'system' | 'user' | 'assistant'; content: string }>,
       temperature: 0.7,
       maxTokens: 4096,
     });