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/user/patch_tests_test.go

@@ -12,6 +12,7 @@ import (
 	"testing"
 
 	"crussell/db"
+	"crussell/testutils"
 	"crussell/mw"
 	"crussell/testutils/fixtures"
 
@@ -50,7 +51,7 @@ func makePatchTestsRequest(handler http.HandlerFunc, method, path string, body i
 // =============================================================================
 
 func TestGetUserPatchTests_Empty(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 	userID, err := fixtures.CreateTestUser(db.DB)
 	if err != nil {
 		t.Fatalf("failed to create user: %v", err)
@@ -71,7 +72,7 @@ func TestGetUserPatchTests_Empty(t *testing.T) {
 }
 
 func TestGetUserPatchTests_WithRecords(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 	userID, err := fixtures.CreateTestUser(db.DB)
 	if err != nil {
 		t.Fatalf("failed to create user: %v", err)
@@ -125,7 +126,7 @@ func TestGetUserPatchTests_InvalidUserID(t *testing.T) {
 // =============================================================================
 
 func TestDeletePatchTest_HappyPath(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 	userID, err := fixtures.CreateTestUser(db.DB)
 	if err != nil {
 		t.Fatalf("failed to create user: %v", err)
@@ -169,7 +170,7 @@ func TestDeletePatchTest_HappyPath(t *testing.T) {
 }
 
 func TestDeletePatchTest_NotFound(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 	userID, err := fixtures.CreateTestUser(db.DB)
 	if err != nil {
 		t.Fatalf("failed to create user: %v", err)