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_definition.go

@@ -0,0 +1,109 @@
+package schema
+
+import (
+	"github.com/Wei-Shaw/sub2api/ent/schema/mixins"
+
+	"entgo.io/ent"
+	"entgo.io/ent/dialect"
+	"entgo.io/ent/dialect/entsql"
+	"entgo.io/ent/schema"
+	"entgo.io/ent/schema/edge"
+	"entgo.io/ent/schema/field"
+	"entgo.io/ent/schema/index"
+)
+
+// UserAttributeDefinition holds the schema definition for custom user attributes.
+//
+// This entity defines the metadata for user attributes, such as:
+//   - Attribute key (unique identifier like "company_name")
+//   - Display name shown in forms
+//   - Field type (text, number, select, etc.)
+//   - Validation rules
+//   - Whether the field is required or enabled
+type UserAttributeDefinition struct {
+	ent.Schema
+}
+
+func (UserAttributeDefinition) Annotations() []schema.Annotation {
+	return []schema.Annotation{
+		entsql.Annotation{Table: "user_attribute_definitions"},
+	}
+}
+
+func (UserAttributeDefinition) Mixin() []ent.Mixin {
+	return []ent.Mixin{
+		mixins.TimeMixin{},
+		mixins.SoftDeleteMixin{},
+	}
+}
+
+func (UserAttributeDefinition) Fields() []ent.Field {
+	return []ent.Field{
+		// key: Unique identifier for the attribute (e.g., "company_name")
+		// Used for programmatic reference
+		field.String("key").
+			MaxLen(100).
+			NotEmpty(),
+
+		// name: Display name shown in forms (e.g., "Company Name")
+		field.String("name").
+			MaxLen(255).
+			NotEmpty(),
+
+		// description: Optional description/help text for the attribute
+		field.String("description").
+			SchemaType(map[string]string{dialect.Postgres: "text"}).
+			Default(""),
+
+		// type: Attribute type - text, textarea, number, email, url, date, select, multi_select
+		field.String("type").
+			MaxLen(20).
+			NotEmpty(),
+
+		// options: Select options for select/multi_select types (stored as JSONB)
+		// Format: [{"value": "xxx", "label": "XXX"}, ...]
+		field.JSON("options", []map[string]any{}).
+			Default([]map[string]any{}).
+			SchemaType(map[string]string{dialect.Postgres: "jsonb"}),
+
+		// required: Whether this attribute is required when editing a user
+		field.Bool("required").
+			Default(false),
+
+		// validation: Validation rules for the attribute value (stored as JSONB)
+		// Format: {"min_length": 1, "max_length": 100, "min": 0, "max": 100, "pattern": "^[a-z]+$", "message": "..."}
+		field.JSON("validation", map[string]any{}).
+			Default(map[string]any{}).
+			SchemaType(map[string]string{dialect.Postgres: "jsonb"}),
+
+		// placeholder: Placeholder text shown in input fields
+		field.String("placeholder").
+			MaxLen(255).
+			Default(""),
+
+		// display_order: Order in which attributes are displayed (lower = first)
+		field.Int("display_order").
+			Default(0),
+
+		// enabled: Whether this attribute is active and shown in forms
+		field.Bool("enabled").
+			Default(true),
+	}
+}
+
+func (UserAttributeDefinition) Edges() []ent.Edge {
+	return []ent.Edge{
+		// values: All user values for this attribute definition
+		edge.To("values", UserAttributeValue.Type),
+	}
+}
+
+func (UserAttributeDefinition) Indexes() []ent.Index {
+	return []ent.Index{
+		// Partial unique index on key (WHERE deleted_at IS NULL) via migration
+		index.Fields("key"),
+		index.Fields("enabled"),
+		index.Fields("display_order"),
+		index.Fields("deleted_at"),
+	}
+}

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"),
+	}
+}