feat: add UMS CLI for binary packaging and system initialization

- Add Cobra-based CLI with ums init, ums serve, ums version commands
- ums init supports interactive prompts and non-interactive flags
- Generates secure JWT secrets and config.yaml automatically
- Extract server.Serve() function for reuse
- Add cross-platform build targets to Makefile
- Update README with CLI installation and usage instructions

New files:
- cmd/ums/main.go - CLI entry point
- cmd/ums/cmd/root.go - Root command
- cmd/ums/cmd/init.go - Interactive/non-interactive init
- cmd/ums/cmd/serve.go - Server command
- cmd/ums/cmd/version.go - Version command
- internal/server/server.go - Extracted Serve function

Makefile

@@ -1,4 +1,4 @@
-.PHONY: help build run test clean vet tidy check run-check db-dir
+.PHONY: help build build-cli build-cli-all run test clean vet tidy check run-check db-dir
 
 help: ## 显示帮助信息
 	@echo "======================================"
@@ -7,6 +7,8 @@ help: ## 显示帮助信息
 	@echo "可用命令:"
 	@echo "  make check       - 全面检查（依赖+vet+编译+测试）"
 	@echo "  make build       - 构建应用"
+	@echo "  make build-cli   - 构建 UMS CLI"
+	@echo "  make build-cli-all - 交叉编译所有平台"
 	@echo "  make run         - 运行应用"
 	@echo "  make test        - 运行测试"
 	@echo "  make vet         - 代码静态检查"
@@ -15,7 +17,17 @@ help: ## 显示帮助信息
 	@echo "  make clean       - 清理构建文件"
 	@echo ""
 
-check: tidy vet build test ## 全面检查：依赖+静态检查+编译+测试
+# CLI 构建配置
+CLI_NAME = ums
+VERSION = 1.0.0
+COMMIT = $(shell git rev-parse --short HEAD 2>/dev/null || echo "dev")
+BUILD_DATE = $(shell date -u '+%Y-%m-%d_%H:%M:%S')
+LDFLAGS = -ldflags "-X github.com/user-management-system/cmd/ums/cmd.Version=$(VERSION) -X github.com/user-management-system/cmd/ums/cmd.Commit=$(COMMIT) -X github.com/user-management-system/cmd/ums/cmd.BuildDate=$(BUILD_DATE)"
+
+# 平台列表
+PLATFORMS = darwin/amd64 darwin/arm64 linux/amd64 linux/arm64 windows/amd64
+
+check: tidy vet build build-cli test ## 全面检查：依赖+静态检查+编译+测试
 
 tidy: ## 整理Go模块依赖
 	@echo "整理依赖..."
@@ -30,6 +42,20 @@ build: db-dir ## 构建应用
 	@echo "构建应用..."
 	go build -o bin/server cmd/server/main.go
 
+build-cli: ## 构建 UMS CLI（当前平台）
+	@echo "构建 UMS CLI..."
+	CGO_ENABLED=0 go build $(LDFLAGS) -o bin/$(CLI_NAME) cmd/ums/main.go
+
+build-cli-all: $(PLATFORMS) ## 构建所有平台的 CLI
+	@echo "所有平台构建完成"
+
+build-cli-%:
+	@platform=$(patsubst %/%,%,$@); \
+	os=$(platform%%/*); \
+	arch=$(platform##*/); \
+	echo "Building for $$os/$$arch"; \
+	CGO_ENABLED=0 GOOS=$$os GOARCH=$$arch go build $(LDFLAGS) -o bin/$(CLI_NAME)-$$os-$$arch cmd/ums/main.go
+
 run: db-dir ## 运行应用
 	@echo "运行应用..."
 	go run cmd/server/main.go