fix(数据层): 修复软删除与唯一约束冲突问题

问题：软删除的记录仍占用唯一约束位置，导致删后无法重建同名/同邮箱/同订阅

修复方案：使用 PostgreSQL 部分唯一索引（WHERE deleted_at IS NULL）
- User.email: 移除字段级 Unique()，改用部分唯一索引
- Group.name: 移除字段级 Unique()，改用部分唯一索引
- UserSubscription.(user_id, group_id): 移除组合唯一索引，改用部分唯一索引
- ApiKey.key: 保留普通唯一约束（安全考虑，已删除的 Key 不应重用）

安全性：
- 应用层已有 ExistsByXxx 检查，自动过滤软删除记录
- 数据库层部分唯一索引提供最后一道防线

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

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

backend/ent/migrate/schema.go

@@ -204,7 +204,7 @@ var (
 		{Name: "created_at", Type: field.TypeTime, SchemaType: map[string]string{"postgres": "timestamptz"}},
 		{Name: "updated_at", Type: field.TypeTime, SchemaType: map[string]string{"postgres": "timestamptz"}},
 		{Name: "deleted_at", Type: field.TypeTime, Nullable: true, SchemaType: map[string]string{"postgres": "timestamptz"}},
-		{Name: "name", Type: field.TypeString, Unique: true, Size: 100},
+		{Name: "name", Type: field.TypeString, Size: 100},
 		{Name: "description", Type: field.TypeString, Nullable: true, SchemaType: map[string]string{"postgres": "text"}},
 		{Name: "rate_multiplier", Type: field.TypeFloat64, Default: 1, SchemaType: map[string]string{"postgres": "decimal(10,4)"}},
 		{Name: "is_exclusive", Type: field.TypeBool, Default: false},
@@ -470,7 +470,7 @@ var (
 		{Name: "created_at", Type: field.TypeTime, SchemaType: map[string]string{"postgres": "timestamptz"}},
 		{Name: "updated_at", Type: field.TypeTime, SchemaType: map[string]string{"postgres": "timestamptz"}},
 		{Name: "deleted_at", Type: field.TypeTime, Nullable: true, SchemaType: map[string]string{"postgres": "timestamptz"}},
-		{Name: "email", Type: field.TypeString, Unique: true, Size: 255},
+		{Name: "email", Type: field.TypeString, Size: 255},
 		{Name: "password_hash", Type: field.TypeString, Size: 255},
 		{Name: "role", Type: field.TypeString, Size: 20, Default: "user"},
 		{Name: "balance", Type: field.TypeFloat64, Default: 0, SchemaType: map[string]string{"postgres": "decimal(20,8)"}},
@@ -605,7 +605,7 @@ var (
 			},
 			{
 				Name:    "usersubscription_user_id_group_id",
-				Unique:  true,
+				Unique:  false,
 				Columns: []*schema.Column{UserSubscriptionsColumns[16], UserSubscriptionsColumns[15]},
 			},
 			{

backend/ent/schema/group.go

@@ -33,10 +33,11 @@ func (Group) Mixin() []ent.Mixin {
 
 func (Group) Fields() []ent.Field {
 	return []ent.Field{
+		// 唯一约束通过部分索引实现（WHERE deleted_at IS NULL），支持软删除后重用
+		// 见迁移文件 016_soft_delete_partial_unique_indexes.sql
 		field.String("name").
 			MaxLen(100).
-			NotEmpty().
-			Unique(),
+			NotEmpty(),
 		field.String("description").
 			Optional().
 			Nillable().

backend/ent/schema/user.go

@@ -33,10 +33,11 @@ func (User) Mixin() []ent.Mixin {
 
 func (User) Fields() []ent.Field {
 	return []ent.Field{
+		// 唯一约束通过部分索引实现（WHERE deleted_at IS NULL），支持软删除后重用
+		// 见迁移文件 016_soft_delete_partial_unique_indexes.sql
 		field.String("email").
 			MaxLen(255).
-			NotEmpty().
-			Unique(),
+			NotEmpty(),
 		field.String("password_hash").
 			MaxLen(255).
 			NotEmpty(),

backend/ent/schema/user_subscription.go

@@ -109,7 +109,9 @@ func (UserSubscription) Indexes() []ent.Index {
 		index.Fields("status"),
 		index.Fields("expires_at"),
 		index.Fields("assigned_by"),
-		index.Fields("user_id", "group_id").Unique(),
+		// 唯一约束通过部分索引实现（WHERE deleted_at IS NULL），支持软删除后重新订阅
+		// 见迁移文件 016_soft_delete_partial_unique_indexes.sql
+		index.Fields("user_id", "group_id"),
 		index.Fields("deleted_at"),
 	}
 }