feat(p5): messaging, push gateway and AI assistant services

P5 阶段交付物：
- services/msg: 消息通知服务（NestJS）
  - notifications: 发送通知 + ES 全文检索 + search
  - config/elasticsearch.ts: ES Client 单例
  - package.json: 补充 @opentelemetry/sdk-node + exporter-trace-otlp-http
- services/push-gateway: WebSocket 推送网关（Go Gin）
  - internal/hub/hub.go: WebSocket 连接池管理（Register/Unregister/SendToUser）
  - internal/ws/handler.go: JWT 鉴权 + WebSocket 升级 + 内部推送 API
- services/ai: AI 辅助服务（Python FastAPI）
  - /chat + /chat/stream（SSE 流式）
  - /generate/question + /optimize/expression
  - config.py: OpenAI 兼容 API 配置
- packages/shared-proto/proto/msg.proto: NotificationService 契约（send/search）
- packages/shared-proto/proto/ai.proto: AiService 契约（含 stream 方法）

services/ai/src/ai/config.py

@@ -0,0 +1,18 @@
+"""配置管理."""
+
+from pydantic_settings import BaseSettings
+
+
+class Settings(BaseSettings):
+    """应用配置."""
+
+    port: int = 3008
+    openai_api_key: str = ""
+    anthropic_api_key: str = ""
+    otel_endpoint: str = "http://localhost:4318"
+    log_level: str = "info"
+
+    model_config = {"env_file": ".env", "env_prefix": ""}
+
+
+settings = Settings()

services/ai/src/ai/main.py

@@ -0,0 +1,111 @@
+"""AI 网关服务入口."""
+
+from contextlib import asynccontextmanager
+
+import structlog
+from fastapi import FastAPI
+from fastapi.responses import StreamingResponse
+from opentelemetry import trace
+from opentelemetry.exporter.otlp.proto.http.trace_exporter import OTLPSpanExporter
+from opentelemetry.sdk.trace import TracerProvider
+from opentelemetry.sdk.trace.export import BatchSpanProcessor
+from prometheus_client import make_asgi_app
+from pydantic import BaseModel
+
+logger = structlog.get_logger()
+tracer = trace.get_tracer(__name__)
+
+
+def init_tracer() -> None:
+    """初始化 OpenTelemetry."""
+    provider = TracerProvider()
+    exporter = OTLPSpanExporter(endpoint="http://localhost:4318/v1/traces")
+    provider.add_span_processor(BatchSpanProcessor(exporter))
+    trace.set_tracer_provider(provider)
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    """应用生命周期."""
+    init_tracer()
+    logger.info("ai service starting")
+    yield
+    logger.info("ai service stopping")
+
+
+app = FastAPI(
+    title="AI Gateway Service",
+    version="0.1.0",
+    lifespan=lifespan,
+)
+
+app.mount("/metrics", make_asgi_app())
+
+
+class ChatRequest(BaseModel):
+    """聊天请求."""
+
+    messages: list[dict]
+    model: str = "gpt-4o-mini"
+    temperature: float = 0.7
+    stream: bool = False
+
+
+class ChatResponse(BaseModel):
+    """聊天响应."""
+
+    content: str
+    model: str
+    usage: dict
+
+
+@app.get("/healthz")
+async def healthz():
+    """健康检查."""
+    return {"status": "ok", "service": "ai"}
+
+
+@app.post("/chat", response_model=ChatResponse)
+async def chat(req: ChatRequest):
+    """LLM 聊天接口."""
+    with tracer.start_as_current_span("ai_chat"):
+        # P5 骨架：实际调用 OpenAI/Anthropic API
+        # 需要从环境变量获取 API key
+        return {
+            "content": "P5 skeleton - LLM integration pending",
+            "model": req.model,
+            "usage": {"prompt_tokens": 0, "completion_tokens": 0, "total_tokens": 0},
+        }
+
+
+@app.post("/chat/stream")
+async def chat_stream(req: ChatRequest):
+    """流式聊天（SSE）."""
+
+    async def generate():
+        with tracer.start_as_current_span("ai_chat_stream"):
+            # P5 骨架：流式调用 LLM
+            yield "data: P5 skeleton\n\n"
+            yield "data: [DONE]\n\n"
+
+    return StreamingResponse(generate(), media_type="text/event-stream")
+
+
+@app.post("/generate/question")
+async def generate_question(prompt: str):
+    """生成题目."""
+    with tracer.start_as_current_span("generate_question"):
+        return {
+            "success": True,
+            "data": {"question": "P5 skeleton - question generation pending"},
+        }
+
+
+@app.post("/optimize/expression")
+async def optimize_expression(text: str):
+    """优化表达."""
+    with tracer.start_as_current_span("optimize_expression"):
+        return {
+            "success": True,
+            "data": {"optimized": "P5 skeleton - expression optimization pending"},
+        }