Support custom OpenAI base URL

[WangNingkai]

Feature Request: Support custom OpenAI base URL

Description

Currently, open-mem only supports the official OpenAI API endpoint (https://api.openai.com/v1). This prevents users from using compatible third-party services like SiliconFlow (硅基流动), DeepSeek, or self-hosted OpenAI-compatible APIs.

Current Behavior

The ModelConfig interface only accepts apiKey, and the OpenAI provider is initialized without a baseURL parameter:
https://github.com/clopca/open-mem/blob/main/src/ai/provider.ts#L64-L69

case "openai": {
  const { createOpenAI } = require("@ai-sdk/openai");
  const openai = createOpenAI({ apiKey: config.apiKey });  // No baseURL option
  return openai(config.model);
}

Expected Behavior

Allow users to specify a custom base URL for OpenAI-compatible providers via:

1. Environment variable: OPENAI_BASE_URL
2. Configuration option in config file

Use Cases

• SiliconFlow (硅基流动): https://api.siliconflow.cn/v1
• DeepSeek: https://api.deepseek.com/v1
• Self-hosted models: Custom local endpoints
• Other OpenAI-compatible services: OpenRouter, Azure OpenAI, etc.

Proposed Solution

1. Update ModelConfig interface (provider.ts)

export interface ModelConfig {
  provider: ProviderType;
  model: string;
  apiKey?: string;
  baseURL?: string;  // Add this
}

2. Pass baseURL to createOpenAI (provider.ts)

case "openai": {
  const { createOpenAI } = require("@ai-sdk/openai");
  const openai = createOpenAI({ 
    apiKey: config.apiKey,
    baseURL: config.baseURL  // Add this
  });
  return openai(config.model);
}

3. Read from environment (config.ts)

Add OPENAI_BASE_URL environment variable support alongside existing OPENAI_API_KEY.

Alternative Considered

If adding baseURL to ModelConfig affects other providers, we could use a generic options field or provider-specific configuration.

Additional Context

The @ai-sdk/openai package already supports baseURL option: https://sdk.vercel.ai/providers/ai-sdk-providers/openai#custom-base-url
This feature would greatly benefit users in regions where OpenAI API access is restricted or expensive, allowing them to use local/cheaper alternatives while maintaining compatibility.