feat(backend): add user custom attributes system

Add a flexible user attribute system that allows admins to define
custom fields for users (text, textarea, number, email, url, date,
select, multi_select types).

- Add Ent schemas for UserAttributeDefinition and UserAttributeValue
- Add service layer with validation logic
- Add repository layer with batch operations support
- Add admin API endpoints for CRUD and reorder operations
- Add batch API for loading attribute values for multiple users
- Add database migration (018_user_attributes.sql)

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

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

backend/ent/schema/user_attribute_value.go

@@ -0,0 +1,74 @@
+package schema
+
+import (
+	"github.com/Wei-Shaw/sub2api/ent/schema/mixins"
+
+	"entgo.io/ent"
+	"entgo.io/ent/dialect/entsql"
+	"entgo.io/ent/schema"
+	"entgo.io/ent/schema/edge"
+	"entgo.io/ent/schema/field"
+	"entgo.io/ent/schema/index"
+)
+
+// UserAttributeValue holds a user's value for a specific attribute.
+//
+// This entity stores the actual values that users have for each attribute definition.
+// Values are stored as strings and converted to the appropriate type by the application.
+type UserAttributeValue struct {
+	ent.Schema
+}
+
+func (UserAttributeValue) Annotations() []schema.Annotation {
+	return []schema.Annotation{
+		entsql.Annotation{Table: "user_attribute_values"},
+	}
+}
+
+func (UserAttributeValue) Mixin() []ent.Mixin {
+	return []ent.Mixin{
+		// Only use TimeMixin, no soft delete - values are hard deleted
+		mixins.TimeMixin{},
+	}
+}
+
+func (UserAttributeValue) Fields() []ent.Field {
+	return []ent.Field{
+		// user_id: References the user this value belongs to
+		field.Int64("user_id"),
+
+		// attribute_id: References the attribute definition
+		field.Int64("attribute_id"),
+
+		// value: The actual value stored as a string
+		// For multi_select, this is a JSON array string
+		field.Text("value").
+			Default(""),
+	}
+}
+
+func (UserAttributeValue) Edges() []ent.Edge {
+	return []ent.Edge{
+		// user: The user who owns this attribute value
+		edge.From("user", User.Type).
+			Ref("attribute_values").
+			Field("user_id").
+			Required().
+			Unique(),
+
+		// definition: The attribute definition this value is for
+		edge.From("definition", UserAttributeDefinition.Type).
+			Ref("values").
+			Field("attribute_id").
+			Required().
+			Unique(),
+	}
+}
+
+func (UserAttributeValue) Indexes() []ent.Index {
+	return []ent.Index{
+		// Unique index on (user_id, attribute_id)
+		index.Fields("user_id", "attribute_id").Unique(),
+		index.Fields("attribute_id"),
+	}
+}