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/repository/repository_test.go

@@ -147,15 +147,36 @@ func TestModelRepository_GetByID(t *testing.T) {
 	assert.Equal(t, "gpt-4", result.ModelName)
 }
 
-func TestModelRepository_GetByModelName(t *testing.T) {
+func TestModelRepository_FindByProviderAndModelName(t *testing.T) {
 	db := setupTestDB(t)
 	repo := NewModelRepository(db)
 
 	repo.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4", Enabled: true})
 
-	result, err := repo.GetByModelName("gpt-4")
+	result, err := repo.FindByProviderAndModelName("p1", "gpt-4")
 	require.NoError(t, err)
 	assert.Equal(t, "m1", result.ID)
+	assert.Equal(t, "p1", result.ProviderID)
+	assert.Equal(t, "gpt-4", result.ModelName)
+}
+
+func TestModelRepository_FindByProviderAndModelName_NotFound(t *testing.T) {
+	db := setupTestDB(t)
+	repo := NewModelRepository(db)
+
+	repo.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4", Enabled: true})
+
+	// Wrong provider_id
+	_, err := repo.FindByProviderAndModelName("p2", "gpt-4")
+	assert.Error(t, err)
+
+	// Wrong model_name
+	_, err = repo.FindByProviderAndModelName("p1", "gpt-3.5")
+	assert.Error(t, err)
+
+	// Both wrong
+	_, err = repo.FindByProviderAndModelName("p2", "claude-3")
+	assert.Error(t, err)
 }
 
 func TestModelRepository_List(t *testing.T) {
@@ -175,6 +196,54 @@ func TestModelRepository_List(t *testing.T) {
 	assert.Len(t, p1Models, 2)
 }
 
+func TestModelRepository_ListEnabled(t *testing.T) {
+	db := setupTestDB(t)
+	providerRepo := NewProviderRepository(db)
+	modelRepo := NewModelRepository(db)
+
+	// Create two providers (both start enabled due to gorm:"default:true")
+	err := providerRepo.Create(&domain.Provider{
+		ID: "enabled-provider", Name: "Enabled Provider",
+		APIKey: "key1", BaseURL: "https://enabled.com", Enabled: true,
+	})
+	require.NoError(t, err)
+	err = providerRepo.Create(&domain.Provider{
+		ID: "disabled-provider", Name: "Disabled Provider",
+		APIKey: "key2", BaseURL: "https://disabled.com", Enabled: true,
+	})
+	require.NoError(t, err)
+
+	// Disable the second provider via Update (GORM default:true skips zero values on Create)
+	err = providerRepo.Update("disabled-provider", map[string]interface{}{"enabled": false})
+	require.NoError(t, err)
+
+	// Create models (all start enabled due to gorm:"default:true")
+	err = modelRepo.Create(&domain.Model{ID: "m1", ProviderID: "enabled-provider", ModelName: "gpt-4", Enabled: true})
+	require.NoError(t, err)
+	err = modelRepo.Create(&domain.Model{ID: "m2", ProviderID: "enabled-provider", ModelName: "gpt-3.5", Enabled: true})
+	require.NoError(t, err)
+	err = modelRepo.Create(&domain.Model{ID: "m3", ProviderID: "disabled-provider", ModelName: "claude-3", Enabled: true})
+	require.NoError(t, err)
+	err = modelRepo.Create(&domain.Model{ID: "m4", ProviderID: "disabled-provider", ModelName: "claude-3.5", Enabled: true})
+	require.NoError(t, err)
+
+	// Disable m2 via Update
+	err = modelRepo.Update("m2", map[string]interface{}{"enabled": false})
+	require.NoError(t, err)
+
+	// ListEnabled should only return models where both model and provider are enabled:
+	// - m1: enabled provider + enabled model -> returned
+	// - m2: enabled provider + disabled model -> filtered out
+	// - m3: disabled provider + enabled model -> filtered out
+	// - m4: disabled provider + enabled model -> filtered out
+	enabled, err := modelRepo.ListEnabled()
+	require.NoError(t, err)
+	require.Len(t, enabled, 1)
+	assert.Equal(t, "m1", enabled[0].ID)
+	assert.Equal(t, "enabled-provider", enabled[0].ProviderID)
+	assert.Equal(t, "gpt-4", enabled[0].ModelName)
+}
+
 func TestModelRepository_Update(t *testing.T) {
 	db := setupTestDB(t)
 	repo := NewModelRepository(db)