引入二级大纲功能，新增父子节点关系及子节点排序，同时调整API与UI以支持嵌套大纲管理

backend/app/services/ai_provider.py

@@ -103,6 +103,65 @@ def build_tool_results_messages(
         ]
 
 
+async def simple_completion(
+    config: AIConfig,
+    messages: list[dict],
+    system_prompt: str = "",
+) -> str:
+    """非流式调用，返回完整文本。用于摘要压缩等场景。"""
+    if config.provider == "anthropic":
+        return await _simple_anthropic(config, messages, system_prompt)
+    return await _simple_openai(config, messages, system_prompt)
+
+
+async def _simple_openai(config: AIConfig, messages: list[dict], system_prompt: str) -> str:
+    url = f"{config.base_url.rstrip('/')}/chat/completions"
+    payload_messages = []
+    if system_prompt:
+        payload_messages.append({"role": "system", "content": system_prompt})
+    payload_messages.extend(messages)
+
+    body = {"model": config.model, "messages": payload_messages, "stream": False}
+    async with httpx.AsyncClient(timeout=120) as client:
+        resp = await client.post(
+            url,
+            headers={"Authorization": f"Bearer {config.api_key}", "Content-Type": "application/json"},
+            json=body,
+        )
+        resp.raise_for_status()
+        data = resp.json()
+        return data["choices"][0]["message"]["content"] or ""
+
+
+async def _simple_anthropic(config: AIConfig, messages: list[dict], system_prompt: str) -> str:
+    url = f"{config.base_url.rstrip('/')}/messages"
+    body: dict = {
+        "model": config.model,
+        "max_tokens": 4096,
+        "messages": messages,
+        "stream": False,
+    }
+    if system_prompt:
+        body["system"] = system_prompt
+
+    async with httpx.AsyncClient(timeout=120) as client:
+        resp = await client.post(
+            url,
+            headers={
+                "x-api-key": config.api_key,
+                "anthropic-version": "2023-06-01",
+                "Content-Type": "application/json",
+            },
+            json=body,
+        )
+        resp.raise_for_status()
+        data = resp.json()
+        for block in data.get("content", []):
+            if block.get("type") == "text":
+                return block["text"]
+        return ""
+
+
 # ── OpenAI 兼容 ──