fix(构建): 删除遗留的 GORM auto_migrate.go 文件

该文件是 GORM 迁移到 Ent ORM 过程中遗留的，仍然导入了
gorm.io/gorm，导致 Docker 构建失败。

文件中的功能已被迁移到 SQL 迁移文件中：
- fixInvalidExpiresAt → 006_fix_invalid_subscription_expires_at.sql
- ensureDefaultGroups → 001_init.sql

🤖 Generated with [Claude Code](https://claude.com/claude-code

)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/repository/auto_migrate.go

-package repository
-
-import (
-	"log"
-	"time"
-
-	"gorm.io/gorm"
-)
-
-// MaxExpiresAt is the maximum allowed expiration date for subscriptions (year 2099)
-// This prevents time.Time JSON serialization errors (RFC 3339 requires year <= 9999)
-var maxExpiresAt = time.Date(2099, 12, 31, 23, 59, 59, 0, time.UTC)
-
-// AutoMigrate runs schema migrations for all repository persistence models.
-// Persistence models are defined within individual `*_repo.go` files.
-func AutoMigrate(db *gorm.DB) error {
-	err := db.AutoMigrate(
-		&userModel{},
-		&apiKeyModel{},
-		&groupModel{},
-		&accountModel{},
-		&accountGroupModel{},
-		&proxyModel{},
-		&redeemCodeModel{},
-		&usageLogModel{},
-		&settingModel{},
-		&userSubscriptionModel{},
-	)
-	if err != nil {
-		return err
-	}
-
-	// 创建默认分组(简易模式支持)
-	if err := ensureDefaultGroups(db); err != nil {
-		return err
-	}
-
-	// 修复无效的过期时间（年份超过 2099 会导致 JSON 序列化失败）
-	return fixInvalidExpiresAt(db)
-}
-
-// fixInvalidExpiresAt 修复 user_subscriptions 表中无效的过期时间
-func fixInvalidExpiresAt(db *gorm.DB) error {
-	result := db.Model(&userSubscriptionModel{}).
-		Where("expires_at > ?", maxExpiresAt).
-		Update("expires_at", maxExpiresAt)
-	if result.Error != nil {
-		return result.Error
-	}
-	if result.RowsAffected > 0 {
-		log.Printf("[AutoMigrate] Fixed %d subscriptions with invalid expires_at (year > 2099)", result.RowsAffected)
-	}
-	return nil
-}
-
-// ensureDefaultGroups 确保默认分组存在(简易模式支持)
-// 为每个平台创建一个默认分组,配置最大权限以确保简易模式下不受限制
-func ensureDefaultGroups(db *gorm.DB) error {
-	defaultGroups := []struct {
-		name        string
-		platform    string
-		description string
-	}{
-		{
-			name:        "anthropic-default",
-			platform:    "anthropic",
-			description: "Default group for Anthropic accounts (Simple Mode)",
-		},
-		{
-			name:        "openai-default",
-			platform:    "openai",
-			description: "Default group for OpenAI accounts (Simple Mode)",
-		},
-		{
-			name:        "gemini-default",
-			platform:    "gemini",
-			description: "Default group for Gemini accounts (Simple Mode)",
-		},
-	}
-
-	for _, dg := range defaultGroups {
-		var count int64
-		if err := db.Model(&groupModel{}).Where("name = ?", dg.name).Count(&count).Error; err != nil {
-			return err
-		}
-
-		if count == 0 {
-			group := &groupModel{
-				Name:             dg.name,
-				Description:      dg.description,
-				Platform:         dg.platform,
-				RateMultiplier:   1.0,
-				IsExclusive:      false,
-				Status:           "active",
-				SubscriptionType: "standard",
-			}
-			if err := db.Create(group).Error; err != nil {
-				log.Printf("[AutoMigrate] Failed to create default group %s: %v", dg.name, err)
-				return err
-			}
-			log.Printf("[AutoMigrate] Created default group: %s (platform: %s)", dg.name, dg.platform)
-		}
-	}
-
-	return nil
-}