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/validators/email_test.go

@@ -189,3 +189,74 @@ func TestParseCursor_EmptyCursor(t *testing.T) {
 		t.Fatal("expected error for empty cursor, got nil")
 	}
 }
+
+func TestParseCursor3_Valid(t *testing.T) {
+	count, tm, id, err := ParseCursor3("5|2026-06-15T10:30:00Z|abc123def456")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if count != 5 {
+		t.Errorf("expected count 5, got %d", count)
+	}
+	if tm.Year() != 2026 || tm.Month() != 6 || tm.Day() != 15 {
+		t.Errorf("unexpected time: %v", tm)
+	}
+	if id != "abc123def456" {
+		t.Errorf("expected id 'abc123def456', got %q", id)
+	}
+}
+
+func TestParseCursor3_InvalidFormat(t *testing.T) {
+	_, _, _, err := ParseCursor3("not-a-valid-cursor")
+	if err == nil {
+		t.Fatal("expected error for invalid cursor format, got nil")
+	}
+}
+
+func TestParseCursor3_InvalidCount(t *testing.T) {
+	_, _, _, err := ParseCursor3("not-a-number|2026-06-15T10:30:00Z|abc123def456")
+	if err == nil {
+		t.Fatal("expected error for invalid count, got nil")
+	}
+}
+
+func TestParseCursor3_InvalidTimestamp(t *testing.T) {
+	_, _, _, err := ParseCursor3("5|not-a-time|abc123def456")
+	if err == nil {
+		t.Fatal("expected error for invalid timestamp, got nil")
+	}
+}
+
+func TestParseCursor3_EmptyCursor(t *testing.T) {
+	_, _, _, err := ParseCursor3("")
+	if err == nil {
+		t.Fatal("expected error for empty cursor, got nil")
+	}
+}
+
+func TestParseCursor3_ZeroCount(t *testing.T) {
+	count, _, _, err := ParseCursor3("0|2026-06-15T10:30:00Z|abc123def456")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if count != 0 {
+		t.Errorf("expected count 0, got %d", count)
+	}
+}
+
+func TestParseCursor3_LargeCount(t *testing.T) {
+	count, _, _, err := ParseCursor3("999|2026-06-15T10:30:00Z|abc123def456")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if count != 999 {
+		t.Errorf("expected count 999, got %d", count)
+	}
+}
+
+func TestParseCursor3_NegativeCount(t *testing.T) {
+	_, _, _, err := ParseCursor3("-1|2026-06-15T10:30:00Z|abc123def456")
+	if err != nil {
+		t.Fatalf("unexpected error for negative count: %v", err)
+	}
+}