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/service/service_supplemental_test.go

@@ -13,7 +13,8 @@ import (
 func TestProviderService_Update(t *testing.T) {
 	db := setupServiceTestDB(t)
 	repo := repository.NewProviderRepository(db)
-	svc := NewProviderService(repo)
+	modelRepo := repository.NewModelRepository(db)
+	svc := NewProviderService(repo, modelRepo)
 
 	svc.Create(&domain.Provider{ID: "p1", Name: "Original", APIKey: "key", BaseURL: "https://test.com"})
 
@@ -28,7 +29,8 @@ func TestProviderService_Update(t *testing.T) {
 func TestProviderService_Update_NotFound(t *testing.T) {
 	db := setupServiceTestDB(t)
 	repo := repository.NewProviderRepository(db)
-	svc := NewProviderService(repo)
+	modelRepo := repository.NewModelRepository(db)
+	svc := NewProviderService(repo, modelRepo)
 
 	err := svc.Update("nonexistent", map[string]interface{}{"name": "test"})
 	assert.Error(t, err)
@@ -41,11 +43,12 @@ func TestModelService_Get(t *testing.T) {
 	svc := NewModelService(modelRepo, providerRepo)
 
 	providerRepo.Create(&domain.Provider{ID: "p1", Name: "P1", APIKey: "key", BaseURL: "https://test.com"})
-	svc.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4"})
+	model := &domain.Model{ProviderID: "p1", ModelName: "gpt-4"}
+	require.NoError(t, svc.Create(model))
 
-	model, err := svc.Get("m1")
+	result, err := svc.Get(model.ID)
 	require.NoError(t, err)
-	assert.Equal(t, "gpt-4", model.ModelName)
+	assert.Equal(t, "gpt-4", result.ModelName)
 }
 
 func TestModelService_Update(t *testing.T) {
@@ -55,14 +58,15 @@ func TestModelService_Update(t *testing.T) {
 	svc := NewModelService(modelRepo, providerRepo)
 
 	providerRepo.Create(&domain.Provider{ID: "p1", Name: "P1", APIKey: "key", BaseURL: "https://test.com"})
-	svc.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4"})
+	model := &domain.Model{ProviderID: "p1", ModelName: "gpt-4"}
+	require.NoError(t, svc.Create(model))
 
-	err := svc.Update("m1", map[string]interface{}{"model_name": "gpt-4o"})
+	err := svc.Update(model.ID, map[string]interface{}{"model_name": "gpt-4o"})
 	require.NoError(t, err)
 
-	model, err := svc.Get("m1")
+	result, err := svc.Get(model.ID)
 	require.NoError(t, err)
-	assert.Equal(t, "gpt-4o", model.ModelName)
+	assert.Equal(t, "gpt-4o", result.ModelName)
 }
 
 func TestModelService_Update_ProviderID_Invalid(t *testing.T) {
@@ -72,9 +76,10 @@ func TestModelService_Update_ProviderID_Invalid(t *testing.T) {
 	svc := NewModelService(modelRepo, providerRepo)
 
 	providerRepo.Create(&domain.Provider{ID: "p1", Name: "P1", APIKey: "key", BaseURL: "https://test.com"})
-	svc.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4"})
+	model := &domain.Model{ProviderID: "p1", ModelName: "gpt-4"}
+	require.NoError(t, svc.Create(model))
 
-	err := svc.Update("m1", map[string]interface{}{"provider_id": "nonexistent"})
+	err := svc.Update(model.ID, map[string]interface{}{"provider_id": "nonexistent"})
 	assert.Error(t, err)
 }
 
@@ -85,12 +90,13 @@ func TestModelService_Delete(t *testing.T) {
 	svc := NewModelService(modelRepo, providerRepo)
 
 	providerRepo.Create(&domain.Provider{ID: "p1", Name: "P1", APIKey: "key", BaseURL: "https://test.com"})
-	svc.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4"})
+	model := &domain.Model{ProviderID: "p1", ModelName: "gpt-4"}
+	require.NoError(t, svc.Create(model))
 
-	err := svc.Delete("m1")
+	err := svc.Delete(model.ID)
 	require.NoError(t, err)
 
-	_, err = svc.Get("m1")
+	_, err = svc.Get(model.ID)
 	assert.Error(t, err)
 }