feat: ReAct Agent 自主科研引擎、LLM 流式工具调用、论文搜索服务化

核心新增 —— ReAct 智能体引擎 (src/agent/):
 - ReAct 主循环：自主推理→工具调用→观察→迭代，支持流式 SSE 推送、
   上下文自动压缩、重复调用死循环检测、手动取消与超时控制
 - 7 个内置工具：ADS/arXiv 论文搜索、元数据查询、全文读取、PDF/HTML
   下载解析、RAG 向量语义检索、CDS Sesame 天体目标查询

 LLM 客户端增强 (src/clients/llm.rs):
 - 新增流式/非流式 Function Calling (tool calling) 支持
 - 原生 reasoning_content 推理链字段 (DeepSeek/QwQ)
 - 多模态图片+文本对话、TokenUsage 用量统计

 搜索服务重构 (src/services/search.rs):
 - 抽取 ADS/arXiv 搜索为统一 service，跨平台去重、本地状态合并
 - API handler 瘦身至薄封装层

 下载器增强 (src/services/download.rs):
 - Obscura 无头浏览器反爬下载通道 (进程内/CLI 双模式)
 - arXiv/Springer 专用策略、ADS Scan PDF 兜底、CAPTCHA 绕过

 解析器增强 (src/services/parser/mod.rs):
 - Markdown 输出自动附加 YAML 前端元数据头 (标题/作者/来源)

 新增 API (src/api/agent.rs):
 - POST /api/chat/agent — SSE 流式智能体对话
 - GET/DELETE /api/chat/sessions — 会话管理 CRUD
 - POST /api/chat/sessions/:id/stop — 紧急停止

 数据库: agent_sessions + agent_messages 表

 前端 (dashboard/):
 - 全新"智能科研"标签页 (ResearchAgentPanel) — 会话列表、ReAct
   步骤时间线可视化、SSE 实时流式渲染、Markdown+KaTeX 答案展示
 - Reader AI 助手集成 Agent 工作流，含工具调用步骤展示与引用溯源
 - 侧边栏新增"智能科研"导航入口

Cargo.lock

@@ -132,6 +132,8 @@ name = "astroresearch"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-stream",
+ "async-trait",
  "axum",
  "base64 0.22.1",
  "chrono",
@@ -177,6 +179,28 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "async-stream"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b5a71a6f37880a80d1d7f19efd781e4b5de42c88f0722cc13bcb6cc2cfe8476"
+dependencies = [
+ "async-stream-impl",
+ "futures-core",
+ "pin-project-lite",
+]
+
+[[package]]
+name = "async-stream-impl"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c7c24de15d275a1ecfd47a380fb4d5ec9bfe0933f309ed5e705b775596a3574d"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "async-trait"
 version = "0.1.89"