feat(ai): add OpenAI client with custom base URL support

TDD implementation of OpenAIClient that wraps the OpenAI chat completions
API with configurable base URL (supports OpenRouter and other compatible APIs).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/ai/__tests__/openai-client.test.ts

@@ -0,0 +1,88 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { OpenAIClient } from "../openai-client";
+
+describe("OpenAIClient", () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("sends correct request format", async () => {
+    const mockFetch = vi.fn().mockResolvedValue({
+      ok: true,
+      json: () =>
+        Promise.resolve({
+          choices: [{ message: { content: '{"summary":"test"}' } }],
+          usage: { total_tokens: 42 },
+        }),
+    });
+    vi.stubGlobal("fetch", mockFetch);
+
+    const client = new OpenAIClient({
+      apiKey: "sk-test-key",
+      model: "gpt-4o-mini",
+      baseUrl: "https://api.openai.com/v1",
+    });
+
+    const result = await client.generateCompletion({
+      systemPrompt: "You are a doctor.",
+      userPrompt: "Analyze this data.",
+    });
+
+    expect(result.content).toBe('{"summary":"test"}');
+    expect(result.tokensUsed).toBe(42);
+    expect(result.model).toBe("gpt-4o-mini");
+    expect(result.providerType).toBe("admin-key");
+
+    const call = mockFetch.mock.calls[0];
+    expect(call[0]).toBe("https://api.openai.com/v1/chat/completions");
+    const body = JSON.parse(call[1].body);
+    expect(body.model).toBe("gpt-4o-mini");
+    expect(body.messages).toHaveLength(2);
+    expect(body.response_format).toEqual({ type: "json_object" });
+  });
+
+  it("throws on non-ok response", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn().mockResolvedValue({
+        ok: false,
+        status: 429,
+        text: () => Promise.resolve("rate limited"),
+      }),
+    );
+
+    const client = new OpenAIClient({
+      apiKey: "sk-test",
+      model: "gpt-4o-mini",
+      baseUrl: "https://api.openai.com/v1",
+    });
+
+    await expect(
+      client.generateCompletion({ systemPrompt: "test", userPrompt: "test" }),
+    ).rejects.toThrow("OpenAI request failed (429)");
+  });
+
+  it("uses custom base URL for OpenRouter", async () => {
+    const mockFetch = vi.fn().mockResolvedValue({
+      ok: true,
+      json: () =>
+        Promise.resolve({
+          choices: [{ message: { content: '{"test":true}' } }],
+          usage: { total_tokens: 10 },
+        }),
+    });
+    vi.stubGlobal("fetch", mockFetch);
+
+    const client = new OpenAIClient({
+      apiKey: "sk-or-test",
+      model: "openai/gpt-4o-mini",
+      baseUrl: "https://openrouter.ai/api/v1",
+    });
+
+    await client.generateCompletion({ systemPrompt: "test", userPrompt: "test" });
+
+    expect(mockFetch.mock.calls[0][0]).toBe(
+      "https://openrouter.ai/api/v1/chat/completions",
+    );
+  });
+});

src/lib/ai/openai-client.ts

@@ -0,0 +1,57 @@
+import type { AIProvider, CompletionParams, CompletionResult } from "./types";
+
+interface OpenAIClientConfig {
+  apiKey: string;
+  model: string;
+  baseUrl: string;
+}
+
+export class OpenAIClient implements AIProvider {
+  readonly type = "admin-key" as const;
+  private config: OpenAIClientConfig;
+
+  constructor(config: OpenAIClientConfig) {
+    this.config = config;
+  }
+
+  async generateCompletion(params: CompletionParams): Promise<CompletionResult> {
+    const url = `${this.config.baseUrl.replace(/\/$/, "")}/chat/completions`;
+
+    const res = await fetch(url, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/json",
+        Authorization: `Bearer ${this.config.apiKey}`,
+      },
+      body: JSON.stringify({
+        model: this.config.model,
+        messages: [
+          { role: "system", content: params.systemPrompt },
+          { role: "user", content: params.userPrompt },
+        ],
+        temperature: params.temperature ?? 0.3,
+        max_tokens: params.maxTokens ?? 1000,
+        response_format: { type: "json_object" },
+      }),
+    });
+
+    if (!res.ok) {
+      const body = await res.text();
+      throw new Error(`OpenAI request failed (${res.status}): ${body}`);
+    }
+
+    const json = await res.json();
+    const content = json.choices?.[0]?.message?.content;
+
+    if (!content) {
+      throw new Error("OpenAI returned empty content");
+    }
+
+    return {
+      content,
+      tokensUsed: json.usage?.total_tokens ?? null,
+      model: this.config.model,
+      providerType: "admin-key",
+    };
+  }
+}