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/bookings_fields_test.go

@@ -9,13 +9,14 @@ import (
 	"testing"
 
 	"crussell/db"
+	"crussell/testutils"
 	"crussell/handlers/bookings"
 	"crussell/testutils/fixtures"
 )
 
 // TestAdminBookings_Get_EnrichedFields verifies that CreatedByName and User.DateOfBirth are populated.
 func TestAdminBookings_Get_EnrichedFields(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	adminID, err := fixtures.CreateTestAdminUser(db.DB)
 	if err != nil {