refactor: full-stack restructure with multi-tenancy, workspace management, and K8s diagnostics

- Add Workspace domain (entity, repository, service, handler, DTO)
- Add multi-tenant K8s client with tenant binding and quota management
- Add K8s diagnostics client (instance diagnostics)
- Add authorization middleware (authz package)
- Restructure frontend to feature-based architecture (features/)
- Add User Management page in configuration
- Add AccessDenied page and route guards
- Refactor shared components (form inputs, layout, UI)
- Update Tailwind config for new design system
- Add comprehensive documentation (docs/, tasks/, plans)
- Improve cluster service with better kubeconfig handling
- Add tests for crypto, config, helm client, tenant binding

backend/Dockerfile

@@ -4,12 +4,17 @@
 # ==================================================
 FROM golang:1.24-alpine AS builder
 
+ARG GOPROXY=https://goproxy.cn,direct
+ARG GOSUMDB=sum.golang.google.cn
+ENV GOPROXY=${GOPROXY}
+ENV GOSUMDB=${GOSUMDB}
+
 RUN apk add --no-cache git make
 
 WORKDIR /build
 
 COPY go.mod go.sum ./
-RUN go mod download
+RUN sh -c 'for i in 1 2 3; do go mod download && exit 0; echo "go mod download failed, retrying ($i/3)" >&2; sleep 5; done; go mod download'
 
 COPY . .
 RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -ldflags="-w -s" -o ocdp-backend cmd/api/main.go