feat(backend): B3 Task 3.2 - SSE 流式响应实现

- 添加 stream_chat 生成器函数处理 SSE 事件流
- 实现 message_start / token / message_end 事件格式
- 添加 messages/stream SSE 端点
- 构建 LLM 消息列表（system prompt + 历史 + 用户消息）
- 持久化用户和 assistant 消息到数据库
- 添加 SSE 测试用 mock Provider

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

backend/app/api/v1/conversations.py

@@ -1,4 +1,5 @@
 from fastapi import APIRouter, Depends, status
+from fastapi.responses import StreamingResponse
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.api.deps import get_db
@@ -53,7 +54,6 @@ async def send_message(
     data: MessageCreate,
     session: AsyncSession = Depends(get_db),
 ) -> MessageResponse:
-    # For now, just create a user message (LLM response will come in Task 3.2)
     await conversation_service.get_conversation(session, conversation_id)
     msg = await conversation_service.create_message(
         session, conversation_id, role="user", content=data.content
@@ -61,6 +61,19 @@ async def send_message(
     return MessageResponse.from_model(msg)
 
 
+@router.post("/{conversation_id}/messages/stream")
+async def send_message_stream(
+    conversation_id: str,
+    data: MessageCreate,
+    session: AsyncSession = Depends(get_db),
+) -> StreamingResponse:
+    """SSE 流式响应端点"""
+    return StreamingResponse(
+        conversation_service.stream_chat(session, conversation_id, data.content),
+        media_type="text/event-stream",
+    )
+
+
 @router.get("/{conversation_id}/messages", response_model=list[MessageResponse])
 async def list_messages(
     conversation_id: str, session: AsyncSession = Depends(get_db)

backend/app/services/conversation_service.py

@@ -1,9 +1,14 @@
+import json
+
 from fastapi import HTTPException
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
-from app.models import Conversation, Message, MessageRole
+from app.models import Conversation, DigitalEmployee, Message, MessageRole
+from app.providers.base import LLMMessage
+from app.services.employee_service import get_employee
+from app.services.tenant_service import get_provider_for_tenant
 
 
 async def create_conversation(
@@ -78,4 +83,67 @@ async def list_messages(session: AsyncSession, conversation_id: str) -> list[Mes
         .where(Message.conversation_id == conversation_id)
         .order_by(Message.created_at)
     )
-    return list(result.scalars().all())
+    return list(result.scalars().all())
+
+
+async def build_messages_for_llm(
+    session: AsyncSession,
+    conversation_id: str,
+    user_content: str,
+) -> tuple[list[LLMMessage], DigitalEmployee]:
+    """构建发送给 LLM 的消息列表，返回 (messages, employee)"""
+    conversation = await get_conversation(session, conversation_id, include_messages=True)
+    employee = await get_employee(session, conversation.employee_id)
+
+    messages: list[LLMMessage] = []
+
+    # System prompt
+    if employee.system_prompt:
+        messages.append(LLMMessage(role="system", content=employee.system_prompt))
+
+    # 历史消息（按 max_context_messages 裁剪）
+    history = conversation.messages or []
+    max_context = employee.max_context_messages
+    if len(history) > max_context:
+        history = history[-max_context:]
+
+    for msg in history:
+        messages.append(LLMMessage(role=msg.role.value, content=msg.content))
+
+    # 当前用户消息
+    messages.append(LLMMessage(role="user", content=user_content))
+
+    return messages, employee
+
+
+async def stream_chat(
+    session: AsyncSession,
+    conversation_id: str,
+    user_content: str,
+):
+    """流式对话生成器，yield SSE 事件字符串"""
+    # 构建消息
+    messages, employee = await build_messages_for_llm(session, conversation_id, user_content)
+
+    # 保存用户消息
+    await create_message(session, conversation_id, MessageRole.user, user_content)
+
+    # 获取 Provider
+    provider = await get_provider_for_tenant(session, employee.tenant_id)
+
+    # 发送 message_start 事件
+    yield f"data: {json.dumps({'type': 'message_start'})}\n\n"
+
+    # 流式生成
+    full_content = ""
+    token_count = 0
+    async for token in provider.chat_stream(messages):
+        full_content += token
+        token_count += 1
+        yield f"data: {json.dumps({'type': 'token', 'content': token})}\n\n"
+
+    # 保存 assistant 消息
+    await create_message(session, conversation_id, MessageRole.assistant, full_content, token_count=token_count)
+
+    # 发送 message_end 事件
+    yield f"data: {json.dumps({'type': 'message_end', 'token_count': token_count})}\n\n"

backend/tests/test_conversations.py

@@ -8,7 +8,17 @@ from app.main import app
 async def tenant_id(async_client: AsyncClient) -> str:
     """Create a tenant and return its ID."""
     resp = await async_client.post("/api/v1/tenants", json={"name": "Test Tenant", "slug": "test-tenant"})
-    return resp.json()["id"]
+    tid = resp.json()["id"]
+    # Configure tenant with LLM provider (mock settings)
+    await async_client.put(
+        f"/api/v1/tenants/{tid}/config",
+        json={
+            "llm_provider": "openai",
+            "llm_api_key": "test-api-key",
+            "llm_model": "gpt-4",
+        },
+    )
+    return tid
 
 
 @pytest.fixture
@@ -186,3 +196,101 @@ class TestMessageOperations:
         assert resp.status_code == 200
         # The response may or may not include messages depending on schema
         # For now, we just verify the conversation is accessible
+
+
+class TestSSEStreaming:
+    """Test SSE streaming message responses."""
+
+    @pytest.fixture
+    async def conversation_id(self, async_client: AsyncClient, tenant_id: str, employee_id: str) -> str:
+        """Create a conversation and return its ID."""
+        resp = await async_client.post(
+            "/api/v1/conversations",
+            json={
+                "tenant_id": tenant_id,
+                "employee_id": employee_id,
+                "user_id": "user-001",
+            },
+        )
+        return resp.json()["id"]
+
+    @pytest.mark.asyncio
+    async def test_sse_stream_events(self, async_client: AsyncClient, conversation_id: str):
+        """Test SSE stream returns properly formatted events."""
+        from unittest.mock import AsyncMock, patch, MagicMock
+
+        # Mock provider that yields tokens
+        async def mock_stream(messages):
+            for token in ["Hello", " ", "world", "!"]:
+                yield token
+
+        mock_provider = MagicMock()
+        mock_provider.chat_stream = mock_stream
+
+        with patch(
+            "app.services.conversation_service.get_provider_for_tenant",
+            AsyncMock(return_value=mock_provider)
+        ):
+            async with async_client.stream(
+                "POST",
+                f"/api/v1/conversations/{conversation_id}/messages/stream",
+                json={"content": "Hello"},
+            ) as response:
+                assert response.status_code == 200
+                assert response.headers["content-type"] == "text/event-stream; charset=utf-8"
+
+                events = []
+                async for line in response.aiter_lines():
+                    if line.startswith("data: "):
+                        events.append(line[6:])  # Remove "data: " prefix
+
+                # Should have at least message_start, some tokens, and message_end
+                assert len(events) >= 2
+
+                # First event should be message_start
+                import json
+                first_event = json.loads(events[0])
+                assert first_event["type"] == "message_start"
+
+                # Last event should be message_end
+                last_event = json.loads(events[-1])
+                assert last_event["type"] == "message_end"
+                assert "token_count" in last_event
+
+    @pytest.mark.asyncio
+    async def test_sse_stream_persists_messages(
+        self, async_client: AsyncClient, conversation_id: str
+    ):
+        """Test that SSE stream persists both user and assistant messages."""
+        from unittest.mock import AsyncMock, patch, MagicMock
+
+        # Mock provider that yields tokens
+        async def mock_stream(messages):
+            for token in ["Test", " ", "response"]:
+                yield token
+
+        mock_provider = MagicMock()
+        mock_provider.chat_stream = mock_stream
+
+        with patch(
+            "app.services.conversation_service.get_provider_for_tenant",
+            AsyncMock(return_value=mock_provider)
+        ):
+            async with async_client.stream(
+                "POST",
+                f"/api/v1/conversations/{conversation_id}/messages/stream",
+                json={"content": "Test question"},
+            ) as response:
+                # Consume the stream
+                async for _ in response.aiter_lines():
+                    pass
+
+        # Verify messages were persisted
+        resp = await async_client.get(f"/api/v1/conversations/{conversation_id}/messages")
+        messages = resp.json()
+
+        # Should have user message and assistant message
+        assert len(messages) >= 2
+        assert messages[0]["role"] == "user"
+        assert messages[0]["content"] == "Test question"
+        assert messages[1]["role"] == "assistant"