fix(security): 修复多个MED安全问题

MED-03: 数据库密码明文配置
- 在 gateway/internal/config/config.go 中添加 AES-GCM 加密支持
- 添加 EncryptedPassword 字段和 GetPassword() 方法
- 支持密码加密存储和解密获取

MED-04: 审计日志Route字段未验证
- 在 supply-api/internal/middleware/auth.go 中添加 sanitizeRoute() 函数
- 防止路径遍历攻击（.., ./, \ 等）
- 防止 null 字节和换行符注入

MED-05: 请求体大小无限制
- 在 gateway/internal/handler/handler.go 中添加 MaxRequestBytes 限制（1MB）
- 添加 maxBytesReader 包装器
- 添加 COMMON_REQUEST_TOO_LARGE 错误码

MED-08: 缺少CORS配置
- 创建 gateway/internal/middleware/cors.go CORS 中间件
- 支持来源域名白名单、通配符子域名
- 支持预检请求处理和凭证配置

MED-09: 错误信息泄露内部细节
- 添加测试验证 JWT 错误消息不包含敏感信息
- 当前实现已正确返回安全错误消息

MED-10: 数据库凭证日志泄露风险
- 在 gateway/cmd/gateway/main.go 中使用 GetPassword() 代替 Password
- 避免 DSN 中明文密码被记录

MED-11: 缺少Token刷新机制
- 当前 verifyToken() 已正确验证 token 过期时间
- Token 刷新需要额外的 refresh token 基础设施

MED-12: 缺少暴力破解保护
- 添加 BruteForceProtection 结构体
- 支持最大尝试次数和锁定时长配置
- 在 TokenVerifyMiddleware 中集成暴力破解保护

gateway/internal/handler/handler_security_test.go

@@ -0,0 +1,118 @@
+package handler
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"lijiaoqiao/gateway/internal/router"
+)
+
+func TestMED05_RequestBodySizeLimit(t *testing.T) {
+	// MED-05: Request body size should be limited to prevent DoS attacks
+	// json.Decoder should use MaxBytes to limit request body size
+
+	r := router.NewRouter(router.StrategyLatency)
+	h := NewHandler(r)
+
+	// Create a very large request body (exceeds 1MB limit)
+	largeContent := strings.Repeat("a", 2*1024*1024) // 2MB
+	largeBody := `{"model": "gpt-4", "messages": [{"role": "user", "content": "` + largeContent + `"}]}`
+
+	req := httptest.NewRequest("POST", "/v1/chat/completions", bytes.NewBufferString(largeBody))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+
+	h.ChatCompletionsHandle(rr, req)
+
+	// After fix: should return 413 Request Entity Too Large
+	if rr.Code != http.StatusRequestEntityTooLarge {
+		t.Errorf("expected status 413 for large request body, got %d", rr.Code)
+	}
+}
+
+func TestMED05_NormalRequestShouldPass(t *testing.T) {
+	// Normal requests should still work
+	r := router.NewRouter(router.StrategyLatency)
+	prov := &mockProvider{name: "test", models: []string{"gpt-4"}, healthy: true}
+	r.RegisterProvider("test", prov)
+
+	h := NewHandler(r)
+
+	body := `{"model": "gpt-4", "messages": [{"role": "user", "content": "hello"}]}`
+	req := httptest.NewRequest("POST", "/v1/chat/completions", bytes.NewBufferString(body))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+
+	h.ChatCompletionsHandle(rr, req)
+
+	// Should succeed (status 200)
+	if rr.Code != http.StatusOK {
+		t.Errorf("expected status 200 for normal request, got %d", rr.Code)
+	}
+}
+
+func TestMED05_EmptyBodyShouldFail(t *testing.T) {
+	// Empty request body should fail
+	r := router.NewRouter(router.StrategyLatency)
+	h := NewHandler(r)
+
+	req := httptest.NewRequest("POST", "/v1/chat/completions", bytes.NewBufferString(""))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+
+	h.ChatCompletionsHandle(rr, req)
+
+	// Should fail with 400 Bad Request
+	if rr.Code != http.StatusBadRequest {
+		t.Errorf("expected status 400 for empty body, got %d", rr.Code)
+	}
+}
+
+func TestMED05_InvalidJSONShouldFail(t *testing.T) {
+	// Invalid JSON should fail
+	r := router.NewRouter(router.StrategyLatency)
+	h := NewHandler(r)
+
+	body := `{invalid json}`
+	req := httptest.NewRequest("POST", "/v1/chat/completions", bytes.NewBufferString(body))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+
+	h.ChatCompletionsHandle(rr, req)
+
+	// Should fail with 400 Bad Request
+	if rr.Code != http.StatusBadRequest {
+		t.Errorf("expected status 400 for invalid JSON, got %d", rr.Code)
+	}
+}
+
+// TestMaxBytesReaderWrapper tests the MaxBytes reader wrapper behavior
+func TestMaxBytesReaderWrapper(t *testing.T) {
+	// Test that limiting reader works correctly
+	content := "hello world"
+	limitedReader := io.LimitReader(bytes.NewReader([]byte(content)), 5)
+
+	buf := make([]byte, 20)
+	n, err := limitedReader.Read(buf)
+
+	// Should only read 5 bytes
+	if n != 5 {
+		t.Errorf("expected to read 5 bytes, got %d", n)
+	}
+	if err != nil && err != io.EOF {
+		t.Errorf("expected no error or EOF, got %v", err)
+	}
+
+	// Reading again should return 0 with EOF
+	n2, err2 := limitedReader.Read(buf)
+	if n2 != 0 {
+		t.Errorf("expected 0 bytes on second read, got %d", n2)
+	}
+	if err2 != io.EOF {
+		t.Errorf("expected EOF on second read, got %v", err2)
+	}
+}