feat: add DeepSeek as AI provider

- Install @ai-sdk/deepseek package
- Add DeepSeek provider support to lib/ai-providers.ts
- Add DeepSeek configuration to env.example
- Update README.md with DeepSeek in provider list
- Support both default and custom base URL for DeepSeek

package.json

@@ -12,6 +12,7 @@
         "@ai-sdk/amazon-bedrock": "^3.0.62",
         "@ai-sdk/anthropic": "^2.0.44",
         "@ai-sdk/azure": "^2.0.69",
+        "@ai-sdk/deepseek": "^1.0.30",
         "@ai-sdk/google": "^2.0.0",
         "@ai-sdk/openai": "^2.0.19",
         "@ai-sdk/react": "^2.0.22",