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]},
 			},
 			{