refactor: 统一使用 OpenAI 兼容 API，支持自定义 base_url/key/model

- 移除 Gemini 和 Ollama 独立适配，统一使用 ChatOpenAI + base_url
- config.ini 简化为 BASE_URL / API_KEY / MODEL / TEMPERATURE / MAX_RETRIES
- 新增 config.example.ini 示例配置
- 移除 langchain-google-genai / langchain-ollama / pymupdf 依赖
- main.py 新增断点续跑：跳过已有 index.md / index_refined.md
- LLM 请求支持 max_retries 自动重试（默认 3 次）
- 优化 README

llm.py

@@ -5,70 +5,36 @@ import os
 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)
+    api_key = config.get("llm", "API_KEY", fallback=None)
     if not os.environ.get("OPENAI_API_KEY"):
-        if openai_api_key:
-            os.environ["OPENAI_API_KEY"] = openai_api_key
+        if api_key:
+            os.environ["OPENAI_API_KEY"] = api_key
         else:
             raise ValueError(
-                "Error: OPENAI_API_KEY not found in config.ini or environment variables"
+                "Error: API_KEY not found in config.ini or environment variables"
             )
-    return
 
 
-def set_gemini_api_key() -> None:
+def get_base_url() -> str | None:
     config = configparser.ConfigParser()
     config.read("config.ini")
-    google_api_key = config.get("llm", "GOOGLE_API_KEY", fallback=None)
-
-    if not os.environ.get("GOOGLE_API_KEY"):
-        if google_api_key:
-            os.environ["GOOGLE_API_KEY"] = google_api_key
-        else:
-            raise ValueError(
-                "Error: GOOGLE_API_KEY not found in config.ini or environment variables"
-            )
-    return
-
-
-def set_ollama_config() -> None:
-    config = configparser.ConfigParser()
-    config.read("config.ini")
-    ollama_base_url = config.get(
-        "llm", "OLLAMA_BASE_URL", fallback="http://localhost:11434"
-    )
-
-    if not os.environ.get("OLLAMA_BASE_URL"):
-        os.environ["OLLAMA_BASE_URL"] = ollama_base_url
-    return
+    base_url = config.get("llm", "BASE_URL", fallback=None)
+    return base_url or os.environ.get("OPENAI_BASE_URL")
 
 
 def get_model_name() -> str:
     config = configparser.ConfigParser()
     config.read("config.ini")
-    provider = config.get("llm", "PROVIDER", fallback="gemini")
-    if provider == "gemini":
-        return config.get("llm", "GEMINI_MODEL_NAME", fallback="gemini-2.5-flash")
-    elif provider == "ollama":
-        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
+    return config.get("llm", "MODEL", fallback="gpt-4o")
 
 
 def get_temperature() -> float:
     config = configparser.ConfigParser()
     config.read("config.ini")
     return float(config.get("llm", "TEMPERATURE", fallback=0.7))
+
+
+def get_max_retries() -> int:
+    config = configparser.ConfigParser()
+    config.read("config.ini")
+    return int(config.get("llm", "MAX_RETRIES", fallback=3))