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/internal/square/square_dev_test.go

@@ -74,15 +74,21 @@ func TestDevClient_CreateCheckout_PendingThenCompleted(t *testing.T) {
 		t.Error("expected checkout ID to be set")
 	}
 
-	time.Sleep(4 * time.Second)
-
-	completed, err := client.GetCheckout(ctx, result.ID)
+	// Poll until the background goroutine completes — avoids any timing assumptions
+	var completed *PaymentResult
+	for i := 0; i < 20; i++ {
+		completed, err = client.GetCheckout(ctx, result.ID)
+		if err == nil && completed.Status == "COMPLETED" {
+			break
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
 	if err != nil {
 		t.Fatalf("GetCheckout failed: %v", err)
 	}
 
 	if completed.Status != "COMPLETED" {
-		t.Errorf("expected status COMPLETED after wait, got %s", completed.Status)
+		t.Errorf("expected status COMPLETED, got %s", completed.Status)
 	}
 
 	if completed.Amount != 8000 {
@@ -115,9 +121,18 @@ func TestDevClient_CreateCheckout_NoTip(t *testing.T) {
 		t.Errorf("expected status PENDING, got %s", result.Status)
 	}
 
-	time.Sleep(4 * time.Second)
+	if result.ID == "" {
+		t.Error("expected checkout ID to be set")
+	}
 
-	completed, err := client.GetCheckout(ctx, result.ID)
+	var completed *PaymentResult
+	for i := 0; i < 20; i++ {
+		completed, err = client.GetCheckout(ctx, result.ID)
+		if err == nil && completed.Status == "COMPLETED" {
+			break
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
 	if err != nil {
 		t.Fatalf("GetCheckout failed: %v", err)
 	}