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/tests/helpers.go

@@ -3,6 +3,7 @@ package tests
 import (
 	"fmt"
 	"testing"
+	"time"
 
 	"nex/backend/internal/config"
 
@@ -11,26 +12,36 @@ import (
 	"gorm.io/gorm"
 )
 
-// SetupTestDB initializes a temporary SQLite database with auto-migration.
+// SetupTestDB initializes an in-memory SQLite database with auto-migration.
+// Uses :memory: mode with MaxOpenConns(1) to ensure all operations share the
+// same connection, avoiding "database is closed" errors from connection pool.
+// Enables foreign key constraints for SQLite.
 func SetupTestDB(t *testing.T) *gorm.DB {
 	t.Helper()
 
-	dir := t.TempDir()
-	dsn := dir + "/test.db"
-
-	db, err := gorm.Open(sqlite.Open(dsn), &gorm.Config{})
+	db, err := gorm.Open(sqlite.Open(":memory:?_foreign_keys=on"), &gorm.Config{})
 	assert.NoError(t, err, "failed to open test database")
 
+	// 限制为单连接，确保 :memory: 数据库不被连接池丢弃
+	sqlDB, err := db.DB()
+	assert.NoError(t, err, "failed to get underlying sql.DB")
+	sqlDB.SetMaxOpenConns(1)
+	sqlDB.SetConnMaxLifetime(0)
+
 	err = db.AutoMigrate(&config.Provider{}, &config.Model{}, &config.UsageStats{})
 	assert.NoError(t, err, "failed to auto-migrate test database")
 
 	return db
 }
 
-// CleanupTestDB closes the database and removes the temp database file.
+// CleanupTestDB closes the database after a brief delay to allow async
+// goroutines (e.g. stats recording) to finish.
 func CleanupTestDB(t *testing.T, db *gorm.DB) {
 	t.Helper()
 
+	// 等待异步 goroutine（如 statsService.Record）完成
+	time.Sleep(50 * time.Millisecond)
+
 	sqlDB, err := db.DB()
 	assert.NoError(t, err, "failed to get underlying sql.DB")
 
@@ -57,7 +68,8 @@ func CreateTestProvider(t *testing.T, db *gorm.DB, id string) config.Provider {
 }
 
 // CreateTestModel creates a test model and returns it.
-func CreateTestModel(t *testing.T, db *gorm.DB, id string, providerID string, modelName string) config.Model {
+// Does NOT assert on error - returns the model and error for caller to verify.
+func CreateTestModel(t *testing.T, db *gorm.DB, id string, providerID string, modelName string) (config.Model, error) {
 	t.Helper()
 
 	model := config.Model{
@@ -68,7 +80,5 @@ func CreateTestModel(t *testing.T, db *gorm.DB, id string, providerID string, mo
 	}
 
 	err := db.Create(&model).Error
-	assert.NoError(t, err, "failed to create test model")
-
-	return model
+	return model, err
 }