feat: 前后端分离架构 — FastAPI SSE后端 + Vue 3前端

将单体 Streamlit 应用拆分为三层架构：
- api_server.py: FastAPI SSE 流式后端 (端口 8000)
- frontend/: Vue 3 + Vite + Pinia 聊天前端 (端口 5173)
- agent/graph.py: 新增 node_start 回调支持
- 更新启动脚本为三服务模式

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -0,0 +1,145 @@
+/** JSON fetch wrapper + SSE streaming helper. */
+
+const BASE = '/api'
+
+export interface SessionSummary {
+  session_id: string
+  session_name: string
+  created_at: string
+  updated_at: string
+}
+
+export interface SessionData extends SessionSummary {
+  agent_state: Record<string, any>
+}
+
+export interface FileInfo {
+  file_id: string
+  filename: string
+  content_type: string
+  size: number
+}
+
+export interface AgentCompleteData {
+  reason: string
+  intent: string
+  status: string
+  jrxml_length: number
+  error_msg: string
+  natural_explanation: string
+  retry_count: number
+  ocr_extraction_result: any
+}
+
+export interface SSECallbacks {
+  onNodeStart?: (data: { node: string; label: string }) => void
+  onNodeComplete?: (data: { node: string; label: string; detail: string }) => void
+  onStreamToken?: (data: { text: string; type: string }) => void
+  onAgentComplete?: (data: AgentCompleteData) => void
+  onAgentError?: (data: { error: string; traceback?: string }) => void
+}
+
+export const api = {
+  // ── Health ──
+  async health() {
+    const r = await fetch(`${BASE}/health`)
+    return r.json()
+  },
+
+  async config() {
+    const r = await fetch(`${BASE}/config`)
+    return r.json()
+  },
+
+  // ── Sessions ──
+  async createSession(): Promise<SessionSummary> {
+    const r = await fetch(`${BASE}/sessions`, { method: 'POST' })
+    return r.json()
+  },
+
+  async listSessions(): Promise<SessionSummary[]> {
+    const r = await fetch(`${BASE}/sessions`)
+    const data = await r.json()
+    return data.sessions
+  },
+
+  async getSession(sessionId: string): Promise<SessionData> {
+    const r = await fetch(`${BASE}/sessions/${sessionId}`)
+    if (!r.ok) throw new Error('会话不存在')
+    return r.json()
+  },
+
+  async deleteSession(sessionId: string): Promise<void> {
+    await fetch(`${BASE}/sessions/${sessionId}`, { method: 'DELETE' })
+  },
+
+  // ── Upload ──
+  async uploadFile(file: File, sessionId: string): Promise<FileInfo> {
+    const form = new FormData()
+    form.append('file', file)
+    const r = await fetch(`${BASE}/upload?session_id=${encodeURIComponent(sessionId)}`, {
+      method: 'POST',
+      body: form,
+    })
+    if (!r.ok) throw new Error('上传失败')
+    return r.json()
+  },
+
+  // ── Chat (SSE) ──
+  async chat(
+    sessionId: string,
+    text: string,
+    fileIds: string[],
+    callbacks: SSECallbacks,
+  ): Promise<void> {
+    const r = await fetch(`${BASE}/sessions/${sessionId}/chat`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ text, file_ids: fileIds }),
+    })
+
+    if (!r.ok) {
+      const err = await r.json().catch(() => ({ detail: r.statusText }))
+      throw new Error(err.detail || '请求失败')
+    }
+
+    const reader = r.body!.getReader()
+    const decoder = new TextDecoder()
+    let buffer = ''
+    let currentEvent = ''
+
+    while (true) {
+      const { done, value } = await reader.read()
+      if (done) break
+
+      buffer += decoder.decode(value, { stream: true })
+      const lines = buffer.split('\n')
+      buffer = lines.pop() || ''
+
+      for (const line of lines) {
+        if (line.startsWith('event: ')) {
+          currentEvent = line.slice(7).trim()
+        } else if (line.startsWith('data: ')) {
+          const payload = JSON.parse(line.slice(6))
+          switch (currentEvent) {
+            case 'node_start':
+              callbacks.onNodeStart?.(payload)
+              break
+            case 'node_complete':
+              callbacks.onNodeComplete?.(payload)
+              break
+            case 'stream_token':
+              callbacks.onStreamToken?.(payload)
+              break
+            case 'agent_complete':
+              callbacks.onAgentComplete?.(payload)
+              break
+            case 'agent_error':
+              callbacks.onAgentError?.(payload)
+              break
+          }
+        }
+      }
+    }
+  },
+}