feat: 添加结构化日志系统，更新LLM配置与全部文档

新增:
- backend/logger.py — 集中日志模块 (JSON格式 + trace_id + 独立llm.log)
- @log_node / @_log_route 装饰器覆盖17个节点和8个路由

改进:
- backend/llm.py — _LLMLoggingWrapper 自动记录LLM输入输出
- backend/llm.py — API Key优先读ANTHROPIC_API_KEY，模型名改为MiniMax-M2.7
- backend/llm.py — get_llm() 新增caller参数标识调用来源
- backend/validation.py — 新增验证结果/连接失败日志
- backend/session.py — 新增会话创建/删除日志
- app.py — 新增用户交互日志 (输入/执行/异常/会话操作)
- app.py — 提前导入torchvision抑制transformers懒加载报错
- .env.example — 新增LOG_DIR/LOG_LEVEL/ANTHROPIC_API_KEY等配置项
- .gitignore — 新增logs/和db/忽略规则

文档:
- ROADMAP.md — 新增阶段四: 可观测性
- README.md — 补充日志架构/LLM配置/项目结构
- CLAUDE.md — 同步最新配置/日志/MAX_RETRY(3)
- CODE_GUIDE.md — 新增第15章日志系统，更新架构图/LLM/配置

agent/graph.py

@@ -1,5 +1,6 @@
 """LangGraph JRXML 生成代理的状态图定义。"""
 
+import functools
 import os
 from typing import Literal
 
@@ -25,14 +26,41 @@ from agent.nodes import (
     correct_jrxml,
     finalize,
 )
+from backend.logger import get_logger
 
 load_dotenv()
 MAX_RETRY = int(os.getenv("MAX_RETRY", "3"))
 
+_graph_log = get_logger("agent")
+
+
+def _log_route(route_name: str):
+    """装饰器：自动记录路由决策。"""
+    def decorator(func):
+        @functools.wraps(func)
+        def wrapper(state: AgentState, *args, **kwargs):
+            target = func(state, *args, **kwargs)
+            _graph_log.info(
+                f"[路由] {route_name} → {target}",
+                extra={
+                    "route": route_name,
+                    "target": target,
+                    "session_id": state.get("session_id", ""),
+                    "intent": state.get("intent", ""),
+                    "status": state.get("status", ""),
+                    "has_jrxml": bool(state.get("current_jrxml", "").strip()),
+                    "retry_count": state.get("retry_count", 0),
+                },
+            )
+            return target
+        return wrapper
+    return decorator
+
 # ============================================================
 # 路由函数
 # ============================================================
 
+@_log_route("route_by_intent")
 def route_by_intent(state: AgentState) -> Literal[
     "retrieve", "modify_jrxml", "save_session",
     "handle_consult", "handle_undo", "handle_reset"
@@ -59,18 +87,22 @@ def route_by_intent(state: AgentState) -> Literal[
         return "retrieve"
 
 
+@_log_route("route_after_generate")
 def route_after_generate(state: AgentState) -> Literal["save_session"]:
     return "save_session"
 
 
+@_log_route("route_after_modify")
 def route_after_modify(state: AgentState) -> Literal["save_session"]:
     return "save_session"
 
 
+@_log_route("route_after_undo")
 def route_after_undo(state: AgentState) -> Literal["save_session"]:
     return "save_session"
 
 
+@_log_route("route_after_save")
 def route_after_save(state: AgentState) -> Literal["validate", "finalize"]:
     # 预览/导出意图跳过验证，直接完成
     intent = state.get("intent", "")
@@ -79,16 +111,19 @@ def route_after_save(state: AgentState) -> Literal["validate", "finalize"]:
     return "validate"
 
 
+@_log_route("route_after_validate")
 def route_after_validate(state: AgentState) -> Literal["finalize", "explain_error"]:
     if state.get("status") == "pass":
         return "finalize"
     return "explain_error"
 
 
+@_log_route("route_after_explain")
 def route_after_explain(state: AgentState) -> Literal["correct_jrxml"]:
     return "correct_jrxml"
 
 
+@_log_route("route_after_correct")
 def route_after_correct(state: AgentState) -> Literal["validate", "finalize"]:
     retry = state.get("retry_count", 0)
     if retry >= MAX_RETRY: