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`

llm.py

@@ -2,11 +2,30 @@ import configparser
 import os
 
 
-def set_api_key(provider: str) -> None:
+def set_api_key() -> None:
+    config = configparser.ConfigParser()
+    config.read("config.ini")
+    provider = config.get("llm", "PROVIDER", fallback="gemini")
     if provider == "gemini":
         set_gemini_api_key()
     elif provider == "ollama":
         set_ollama_config()
+    elif provider == "openai":
+        set_openai_api_key()
+
+
+def set_openai_api_key() -> None:
+    config = configparser.ConfigParser()
+    config.read("config.ini")
+    openai_api_key = config.get("llm", "OPENAI_API_KEY", fallback=None)
+    if not os.environ.get("OPENAI_API_KEY"):
+        if openai_api_key:
+            os.environ["OPENAI_API_KEY"] = openai_api_key
+        else:
+            raise ValueError(
+                "Error: OPENAI_API_KEY not found in config.ini or environment variables"
+            )
+    return
 
 
 def set_gemini_api_key() -> None:
@@ -41,9 +60,11 @@ def get_model_name() -> str:
     config.read("config.ini")
     provider = config.get("llm", "PROVIDER", fallback="gemini")
     if provider == "gemini":
-        return config.get("llm", "MODEL_NAME", fallback="gemini-2.5-flash")
+        return config.get("llm", "GEMINI_MODEL_NAME", fallback="gemini-2.5-flash")
     elif provider == "ollama":
-        return config.get("llm", "MODEL_NAME", fallback="gemma3:latest")
+        return config.get("llm", "OLLAMA_MODEL_NAME", fallback="gemma3:latest")
+    elif provider == "openai":
+        return config.get("llm", "OPENAI_MODEL_NAME", fallback="gpt-5-mini")
     return "gemini-2.5-flash"  # Default fallback