refactor: 重构项目结构，迁移后端至 mengyastore-backend-go，新增 Java 后端、前端功能更新及部署文档

mengyastore-backend-go/Dockerfile

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+FROM golang:1.24-alpine AS builder
+
+WORKDIR /src
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -o /out/mengyastore-backend .
+
+FROM alpine:3.20
+
+RUN apk add --no-cache ca-certificates tzdata
+
+WORKDIR /app
+
+COPY --from=builder /out/mengyastore-backend ./mengyastore-backend
+
+EXPOSE 8080
+
+ENV GIN_MODE=release
+
+CMD ["./mengyastore-backend"]