feat: Phase 5 — AI Research Agent（受控工具白名单 + LLM 编排 + API）

- agent/tools.py：Tool 元数据（JSON Schema）+ 白名单调用（异常转可读反馈，不中断对话）
- agent/tools_impl.py：6 个受控工具 search_stocks / get_market_data / test_factor / run_backtest / get_experiment / compare_experiments —— 全部只读经 Job/Experiment 链路，研究自动归档；无 shell/任意执行/写删数据能力
- agent/llm.py：LLMClient 抽象 + OpenAI 兼容客户端（LLM_API_KEY/LLM_BASE_URL/LLM_MODEL 走 .env，未配置给出引导提示）+ 研究纪律 system prompt（反过拟合/样本外/成本）
- agent/service.py：编排循环（tool/final JSON 决策 → 执行 → 回喂 → 结论），轮次上限兜底，未知工具拒绝
- /api/agent/chat；httpx 移至主依赖；Job 默认工厂抽取（api/agent/executor 复用）
- 测试 6 项（白名单无 shell、完整研究循环产出、未知工具拒绝、轮次兜底），全量 79 passed / ruff clean

backend/app/api/jobs.py

@@ -14,7 +14,7 @@ from datetime import datetime
 from fastapi import APIRouter, BackgroundTasks, HTTPException
 from fastapi.responses import StreamingResponse
 
-from app.api.deps import DbSession, JobRepoDep, _engine_factory
+from app.api.deps import DbSession, JobRepoDep
 from app.application.services.job_executor import execute_job, new_id
 from app.domain.entities.research import (
     BacktestResult,
@@ -24,27 +24,17 @@ from app.domain.entities.research import (
     ResearchSpec,
 )
 from app.infrastructure.persistence.sqlalchemy.repositories.jobs_impl import (
-    SqlAlchemyExperimentRepository,
     SqlAlchemyJobRepository,
 )
-from app.infrastructure.persistence.sqlalchemy.repositories.market_impl import (
-    SqlAlchemyDailyBarRepository,
-    SqlAlchemyStockRepository,
-)
 from app.infrastructure.persistence.sqlalchemy.session import SessionLocal
 
 router = APIRouter(prefix="/jobs", tags=["jobs"])
 
 
 def _bg_factories():
-    return {
-        "session_factory": SessionLocal,
-        "job_repo_factory": lambda s: SqlAlchemyJobRepository(s),
-        "experiment_repo_factory": lambda s: SqlAlchemyExperimentRepository(s),
-        "stock_repo_factory": lambda s: SqlAlchemyStockRepository(s),
-        "daily_repo_factory": lambda s: SqlAlchemyDailyBarRepository(s),
-        "engine": _engine_factory(),
-    }
+    from app.application.services.job_executor import default_factories
+
+    return default_factories()
 
 
 def _decode_result(kind: str, result_json: str | None):