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/today_test.go

@@ -25,6 +25,7 @@ import (
 	"time"
 
 	"crussell/db"
+	"crussell/testutils"
 	"crussell/handlers/notifications"
 	"crussell/handlers/today"
 	"crussell/mw"
@@ -33,7 +34,7 @@ import (
 // TestAdminToday_CurrentNext verifies that an admin can retrieve the currently
 // in-progress booking and the next upcoming booking for the dashboard.
 func TestAdminToday_CurrentNext(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create test user
 	var userID string
@@ -108,7 +109,7 @@ func TestAdminToday_CurrentNext(t *testing.T) {
 // TestAdminToday_CurrentNext_ClosingTime verifies that the current-next endpoint
 // returns the closing time for today.
 func TestAdminToday_CurrentNext_ClosingTime(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Seed working hours for today (DB uses 0=Monday, 6=Sunday)
 	todayWeekday := int(time.Now().Weekday())
@@ -154,7 +155,7 @@ func TestAdminToday_CurrentNext_ClosingTime(t *testing.T) {
 // TestAdminToday_Appointments tests that an admin can get a list of all
 // bookings scheduled for today with their details.
 func TestAdminToday_Appointments(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create test user
 	var userID string
@@ -229,7 +230,7 @@ func TestAdminToday_Appointments(t *testing.T) {
 // TestAdminToday_PendingApprovals verifies that an admin can see all pending
 // bookings that require approval/confirmation.
 func TestAdminToday_PendingApprovals(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create test user
 	var userID string
@@ -311,7 +312,7 @@ func TestAdminToday_PendingApprovals(t *testing.T) {
 //
 // The transition happens silently in the background during GET requests, not via cron.
 func TestAdminToday_AutoTransition_ConfirmedToInProgress(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create test user
 	var userID string
@@ -384,7 +385,7 @@ func TestAdminToday_AutoTransition_ConfirmedToInProgress(t *testing.T) {
 //
 // The transition happens silently in the background during GET requests, not via cron.
 func TestAdminToday_AutoTransition_InProgressToCompleted(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create test user
 	var userID string
@@ -454,7 +455,7 @@ func TestAdminToday_AutoTransition_InProgressToCompleted(t *testing.T) {
 // TestAdminToday_NoAutoTransition_BeforeStartTime verifies that a confirmed
 // booking that hasn't started yet is NOT transitioned to in_progress.
 func TestAdminToday_NoAutoTransition_BeforeStartTime(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create test user
 	var userID string
@@ -523,7 +524,7 @@ func TestAdminToday_NoAutoTransition_BeforeStartTime(t *testing.T) {
 // TestAdminToday_AutoTransition_CurrentNextHandler verifies that auto-transition
 // also works when calling GetCurrentAndNextHandler (not just appointments handler)
 func TestAdminToday_AutoTransition_CurrentNextHandler(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Create test user
 	var userID string
@@ -611,7 +612,7 @@ func TestAdminToday_AutoTransition_CurrentNextHandler(t *testing.T) {
 //   - total_bookings counts non-cancelled bookings, excluding cancelled/no_show
 //   - The range includes bookings from both the closed day and prior open days
 func TestAdminToday_ClosedDay_Summary(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	ctx := context.Background()
 	now := time.Now()
@@ -732,7 +733,7 @@ func TestAdminToday_ClosedDay_Summary(t *testing.T) {
 //   - summary_scope = "day" (today's summary)
 //   - week_summary is present with summary_scope = "week"
 func TestAdminToday_WeekSummary_TomorrowClosed(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	ctx := context.Background()
 	now := time.Now()
@@ -823,7 +824,7 @@ func TestAdminToday_WeekSummary_TomorrowClosed(t *testing.T) {
 // a closed day, even when default working_hours says today is open.
 // This tests the column name fix: monday_week_start → week_start.
 func TestAdminToday_ExceptionalHours_ClosedDay(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	ctx := context.Background()
 	now := time.Now()
@@ -963,7 +964,7 @@ func TestAdminNotifications_Acknowledge(t *testing.T) {
 // TestAdminToday_NonAdmin verifies that non-admin users receive HTTP 403
 // when accessing today's dashboard endpoints.
 func TestAdminToday_NonAdmin(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	// Test current-next endpoint
 	currentNextHandler := mw.RequireAdmin(http.HandlerFunc(today.GetCurrentAndNextHandler))