feat: 实现统一模型 ID 机制

实现统一模型 ID 格式 (provider_id/model_name)，支持跨协议模型标识和 Smart Passthrough。

核心变更：
- 新增 pkg/modelid 包：解析、格式化、校验统一模型 ID
- 数据库迁移：models 表使用 UUID 主键 + UNIQUE(provider_id, model_name) 约束
- Repository 层：FindByProviderAndModelName、ListEnabled 方法
- Service 层：联合唯一校验、provider ID 字符集校验
- Conversion 层：ExtractModelName、RewriteRequestModelName/RewriteResponseModelName 方法
- Handler 层：统一模型 ID 路由、Smart Passthrough、Models API 本地聚合
- 新增 error-responses、unified-model-id 规范

测试覆盖：
- 单元测试：modelid、conversion、handler、service、repository
- 集成测试：统一模型 ID 路由、Smart Passthrough 保真性、跨协议转换
- 迁移测试：UUID 主键、UNIQUE 约束、级联删除

OpenSpec：
- 归档 unified-model-id 变更到 archive/2026-04-21-unified-model-id
- 同步 11 个 delta specs 到 main specs
- 新增 error-responses、unified-model-id 规范文件

backend/internal/handler/handler_test.go

@@ -13,7 +13,6 @@ import (
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
-	"gorm.io/gorm"
 
 	"nex/backend/internal/domain"
 	"nex/backend/internal/provider"
@@ -31,7 +30,7 @@ type mockRoutingService struct {
 	err    error
 }
 
-func (m *mockRoutingService) Route(modelName string) (*domain.RouteResult, error) {
+func (m *mockRoutingService) RouteByModelName(providerID, modelName string) (*domain.RouteResult, error) {
 	return m.result, m.err
 }
 
@@ -57,6 +56,14 @@ type mockProviderService struct {
 	err       error
 }
 
+func (m *mockProviderService) ListEnabledModels() ([]domain.Model, error) {
+	return nil, nil
+}
+
+func (m *mockProviderService) GetModelByProviderAndName(providerID, modelName string) (*domain.Model, error) {
+	return nil, nil
+}
+
 func (m *mockProviderService) Create(provider *domain.Provider) error { return m.err }
 func (m *mockProviderService) Get(id string, maskKey bool) (*domain.Provider, error) {
 	return m.provider, m.err
@@ -73,13 +80,21 @@ type mockModelService struct {
 	err    error
 }
 
-func (m *mockModelService) Create(model *domain.Model) error { return m.err }
+func (m *mockModelService) Create(model *domain.Model) error {
+	if m.err == nil {
+		model.ID = "mock-uuid-1234"
+	}
+	return m.err
+}
 func (m *mockModelService) Get(id string) (*domain.Model, error) {
 	return m.model, m.err
 }
 func (m *mockModelService) List(providerID string) ([]domain.Model, error) {
 	return m.models, m.err
 }
+func (m *mockModelService) ListEnabled() ([]domain.Model, error) {
+	return []domain.Model{}, nil
+}
 func (m *mockModelService) Update(id string, updates map[string]interface{}) error {
 	return m.err
 }
@@ -163,8 +178,8 @@ func TestModelHandler_CreateModel_MissingFields(t *testing.T) {
 func TestModelHandler_ListModels(t *testing.T) {
 	h := NewModelHandler(&mockModelService{
 		models: []domain.Model{
-			{ID: "m1", ModelName: "gpt-4"},
-			{ID: "m2", ModelName: "gpt-3.5"},
+			{ID: "m1", ProviderID: "openai", ModelName: "gpt-4"},
+			{ID: "m2", ProviderID: "anthropic", ModelName: "claude-3"},
 		},
 	})
 
@@ -174,6 +189,72 @@ func TestModelHandler_ListModels(t *testing.T) {
 
 	h.ListModels(c)
 	assert.Equal(t, 200, w.Code)
+
+	var result []modelResponse
+	require.NoError(t, json.Unmarshal(w.Body.Bytes(), &result))
+	require.Len(t, result, 2)
+	assert.Equal(t, "openai/gpt-4", result[0].UnifiedModelID)
+	assert.Equal(t, "anthropic/claude-3", result[1].UnifiedModelID)
+}
+
+func TestModelHandler_GetModel_UnifiedID(t *testing.T) {
+	h := NewModelHandler(&mockModelService{
+		model: &domain.Model{ID: "m1", ProviderID: "openai", ModelName: "gpt-4"},
+	})
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "m1"}}
+	c.Request = httptest.NewRequest("GET", "/api/models/m1", nil)
+
+	h.GetModel(c)
+	assert.Equal(t, 200, w.Code)
+
+	var result modelResponse
+	require.NoError(t, json.Unmarshal(w.Body.Bytes(), &result))
+	assert.Equal(t, "m1", result.ID)
+	assert.Equal(t, "openai/gpt-4", result.UnifiedModelID)
+}
+
+func TestModelHandler_CreateModel_UnifiedID(t *testing.T) {
+	h := NewModelHandler(&mockModelService{})
+
+	body, _ := json.Marshal(map[string]string{
+		"provider_id": "openai",
+		"model_name":  "gpt-4",
+	})
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest("POST", "/api/models", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	h.CreateModel(c)
+	assert.Equal(t, 201, w.Code)
+
+	var result modelResponse
+	require.NoError(t, json.Unmarshal(w.Body.Bytes(), &result))
+	assert.Equal(t, "mock-uuid-1234", result.ID)
+	assert.Equal(t, "openai/gpt-4", result.UnifiedModelID)
+}
+
+func TestModelHandler_UpdateModel_UnifiedID(t *testing.T) {
+	h := NewModelHandler(&mockModelService{
+		model: &domain.Model{ID: "m1", ProviderID: "openai", ModelName: "gpt-4-turbo"},
+	})
+
+	body, _ := json.Marshal(map[string]interface{}{"enabled": false})
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "m1"}}
+	c.Request = httptest.NewRequest("PUT", "/api/models/m1", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	h.UpdateModel(c)
+	assert.Equal(t, 200, w.Code)
+
+	var result modelResponse
+	require.NoError(t, json.Unmarshal(w.Body.Bytes(), &result))
+	assert.Equal(t, "openai/gpt-4-turbo", result.UnifiedModelID)
 }
 
 // ============ Stats Handler 测试 ============
@@ -256,7 +337,7 @@ func formatMapErrors(errs map[string]string) string {
 
 func TestProviderHandler_CreateProvider_DuplicatedKey(t *testing.T) {
 	h := NewProviderHandler(&mockProviderService{
-		err: gorm.ErrDuplicatedKey,
+		err: appErrors.ErrConflict,
 	})
 
 	body, _ := json.Marshal(map[string]string{