feat: Go 重写后端，替换 Python FastAPI

用 Go (Gin + GORM + SQLite) 重写整个后端：
- 单二进制部署，不依赖 Python/pip/SDK
- net/http 原生客户端，无 Cloudflare TLS 指纹问题
- 多阶段 Dockerfile：Node 构建前端 + Go 构建后端 + Alpine 运行
- 内存占用从 ~95MB 降至 ~3MB
- 完整保留所有 API 路由、JWT 认证、API Key 权限、审计日志

Dockerfile

@@ -0,0 +1,25 @@
+# Stage 1: Build frontend
+FROM node:20-alpine AS frontend-builder
+WORKDIR /app/frontend
+COPY frontend/package*.json ./
+RUN npm install
+COPY frontend/ ./
+RUN npm run build
+
+# Stage 2: Build Go backend
+FROM golang:1.25-alpine AS backend-builder
+WORKDIR /app
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN go build -o airwallex-admin .
+
+# Stage 3: Final minimal image
+FROM alpine:3.19
+RUN apk add --no-cache ca-certificates
+WORKDIR /app
+COPY --from=backend-builder /app/airwallex-admin .
+COPY --from=frontend-builder /app/frontend/dist ./frontend/dist
+RUN mkdir -p /app/data
+EXPOSE 8000
+CMD ["./airwallex-admin"]