auto-claude: subtask-4-2 - Create Deepseek API client wrapper using OpenAI SDK

- Add Deepseek client using OpenAI SDK with custom baseURL
- Implement createDeepseekClient() factory function with lazy initialization
- Add ChatMessage type definitions for type safety
- Include portfolio-specific system prompt for the chatbot
- Support both streaming and non-streaming chat completions
- Configure default model (deepseek-chat) and sensible defaults

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/lib/deepseek.ts

@@ -0,0 +1,92 @@
+import OpenAI from 'openai';
+
+// Deepseek configuration constants
+const DEEPSEEK_BASE_URL = 'https://api.deepseek.com';
+const DEEPSEEK_DEFAULT_MODEL = 'deepseek-chat';
+
+// Create Deepseek client using OpenAI SDK
+// The client is lazy-initialized to avoid issues during build time
+let deepseekClient: OpenAI | null = null;
+
+export function createDeepseekClient(): OpenAI {
+  if (!process.env.DEEPSEEK_API_KEY) {
+    throw new Error('DEEPSEEK_API_KEY environment variable is not set');
+  }
+
+  if (!deepseekClient) {
+    deepseekClient = new OpenAI({
+      baseURL: DEEPSEEK_BASE_URL,
+      apiKey: process.env.DEEPSEEK_API_KEY,
+    });
+  }
+
+  return deepseekClient;
+}
+
+// Type definitions for chat messages
+export type ChatRole = 'system' | 'user' | 'assistant';
+
+export interface ChatMessage {
+  role: ChatRole;
+  content: string;
+}
+
+// System prompt for the portfolio chatbot
+export const PORTFOLIO_SYSTEM_PROMPT = `You are a helpful AI assistant on Damjan Savić's portfolio website.
+Damjan is a full-stack web developer and AI specialist based in Vienna, Austria.
+You can help visitors learn about Damjan's skills, services, and experience.
+Be friendly, professional, and concise in your responses.
+If asked about topics unrelated to Damjan or web development, politely redirect the conversation.
+Respond in the same language the user writes to you (German, English, or Serbian).`;
+
+// Chat completion options
+export interface ChatCompletionOptions {
+  messages: ChatMessage[];
+  model?: string;
+  temperature?: number;
+  maxTokens?: number;
+  stream?: boolean;
+}
+
+// Create a chat completion (non-streaming)
+export async function createChatCompletion(
+  options: ChatCompletionOptions
+): Promise<string> {
+  const client = createDeepseekClient();
+
+  const completion = await client.chat.completions.create({
+    model: options.model ?? DEEPSEEK_DEFAULT_MODEL,
+    messages: options.messages,
+    temperature: options.temperature ?? 0.7,
+    max_tokens: options.maxTokens ?? 1024,
+    stream: false,
+  });
+
+  return completion.choices[0]?.message?.content ?? '';
+}
+
+// Create a streaming chat completion
+export async function createStreamingChatCompletion(
+  options: Omit<ChatCompletionOptions, 'stream'>
+): Promise<AsyncIterable<OpenAI.Chat.Completions.ChatCompletionChunk>> {
+  const client = createDeepseekClient();
+
+  const stream = await client.chat.completions.create({
+    model: options.model ?? DEEPSEEK_DEFAULT_MODEL,
+    messages: options.messages,
+    temperature: options.temperature ?? 0.7,
+    max_tokens: options.maxTokens ?? 1024,
+    stream: true,
+  });
+
+  return stream;
+}
+
+// Default export for convenience
+export default {
+  createClient: createDeepseekClient,
+  createCompletion: createChatCompletion,
+  createStreamingCompletion: createStreamingChatCompletion,
+  SYSTEM_PROMPT: PORTFOLIO_SYSTEM_PROMPT,
+  DEFAULT_MODEL: DEEPSEEK_DEFAULT_MODEL,
+};