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/bookings/admin_reserve_test.go

@@ -20,6 +20,7 @@ import (
 	"time"
 
 	"crussell/db"
+	"crussell/testutils"
 	"crussell/mw"
 	"crussell/testutils/fixtures"
 
@@ -59,7 +60,7 @@ func makeAdminReserveRequest(handler http.Handler, body interface{}, adminID str
 // create a walk-in reservation with a valid duration. The test verifies
 // the reservation is created in the database with the correct duration.
 func TestAdminReserveSlot_WalkIn_Success(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	seedDefaultWorkingHours(t)
 
@@ -117,7 +118,7 @@ func TestAdminReserveSlot_WalkIn_Success(t *testing.T) {
 // create a call-in reservation with valid service IDs. The test verifies
 // the reservation duration matches the service duration.
 func TestAdminReserveSlot_CallIn_Success(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	seedDefaultWorkingHours(t)
 
@@ -193,7 +194,7 @@ func TestAdminReserveSlot_CallIn_Success(t *testing.T) {
 // TestAdminReserveSlot_WalkIn_MissingDuration tests that walk-in reservations
 // fail with HTTP 400 when duration_minutes is missing or zero.
 func TestAdminReserveSlot_WalkIn_MissingDuration(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	seedDefaultWorkingHours(t)
 
@@ -231,7 +232,7 @@ func TestAdminReserveSlot_WalkIn_MissingDuration(t *testing.T) {
 // TestAdminReserveSlot_CallIn_MissingServices tests that call-in reservations
 // fail with HTTP 400 when service_ids is empty.
 func TestAdminReserveSlot_CallIn_MissingServices(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	seedDefaultWorkingHours(t)
 
@@ -272,7 +273,7 @@ func TestAdminReserveSlot_CallIn_MissingServices(t *testing.T) {
 // TestAdminReserveSlot_InvalidReservationType tests that reservations
 // fail with HTTP 400 when reservation_type is invalid.
 func TestAdminReserveSlot_InvalidReservationType(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	seedDefaultWorkingHours(t)
 
@@ -309,7 +310,7 @@ func TestAdminReserveSlot_InvalidReservationType(t *testing.T) {
 // TestAdminReserveSlot_SlotOverlap tests that a reservation fails
 // with HTTP 409 when the slot overlaps with an existing booking.
 func TestAdminReserveSlot_SlotOverlap(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	seedDefaultWorkingHours(t)
 
@@ -381,7 +382,7 @@ func TestAdminReserveSlot_SlotOverlap(t *testing.T) {
 // TestAdminReserveSlot_ReplacesExisting tests that reserving twice
 // on the same admin replaces the previous reservation.
 func TestAdminReserveSlot_ReplacesExisting(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	seedDefaultWorkingHours(t)
 
@@ -469,7 +470,7 @@ func TestAdminReserveSlot_ReplacesExisting(t *testing.T) {
 
 // TestAdminReserveSlot_WalkIn_PastStart tests that walk-in reservations
 func TestAdminReserveSlot_WalkIn_PastStart(t *testing.T) {
-	resetTestData(t)
+	testutils.SetupTestDB(t)
 
 	seedDefaultWorkingHours(t)