feat: Introduce OpenAI LLM provider and update API key handling

This commit integrates OpenAI as a new Large Language Model (LLM) provider,
expanding the available options for content refinement.

Key changes include:
- Added `set_openai_api_key` to handle OpenAI API key retrieval from
  `config.ini` or environment variables.
- Modified `set_api_key` to dynamically read the LLM provider from `config.ini`

pyproject.toml

@@ -10,5 +10,6 @@ dependencies = [
     "langchain-community>=0.4.1",
     "langchain-google-genai>=3.0.0",
     "langchain-ollama>=1.0.0",
+    "langchain-openai>=1.0.2",
     "pymupdf>=1.26.6",
 ]