refactor(backend): replace resetTestData with SetupTestDB and add new tests

Migrate all test files from resetTestData(t) to testutils.SetupTestDB(t) for isolated per-package test databases.

- Add new feature tests: name history assertions, referral discount preview,
  time blockers, email validation, GDPR export, loyalty manual redemption
- Update existing tests to use batch queries and SetupTestDB
- Remove test_helpers.go resetTestData infrastructure
- Add comprehensive user profile tests (442 new lines)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/admin/services_test.go

@@ -20,6 +20,7 @@ import (
 	"testing"
 
 	"crussell/db"
+	"crussell/testutils"
 	"crussell/handlers/services"
 	"crussell/mw"
 )
@@ -28,7 +29,7 @@ import (
 // with name, description, price, duration, and minimum age requirements. The new
 // service is active by default and stored in the database.
 func TestAdminServices_Create(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create admin user in DB first
 	_, err := db.DB.Exec(context.Background(), `
@@ -74,7 +75,7 @@ func TestAdminServices_Create(t *testing.T) {
 // TestAdminServices_List tests that an admin can retrieve all services,
 // including inactive ones. This is useful for managing the full service catalog.
 func TestAdminServices_List(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Insert test services
 	_, err := db.DB.Exec(context.Background(), `
@@ -125,7 +126,7 @@ func TestAdminServices_List(t *testing.T) {
 // active status on/off. This is used to temporarily disable a service without
 // deleting it from the system.
 func TestAdminServices_Toggle(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create a service
 	var serviceID string
@@ -175,7 +176,7 @@ func TestAdminServices_Toggle(t *testing.T) {
 // by setting is_active to false. The service record remains but is hidden from
 // customers.
 func TestAdminServices_Delete(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create a service
 	var serviceID string
@@ -210,7 +211,7 @@ func TestAdminServices_Delete(t *testing.T) {
 // Forbidden when attempting to create, list, toggle, or delete services. This
 // ensures proper role-based access control.
 func TestAdminServices_NonAdmin(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create regular user in DB
 	_, err := db.DB.Exec(context.Background(), `