refactor(backend): migrate from sql.ErrNoRows to pgx.ErrNoRows

Replace all database/sql.ErrNoRows checks with pgx.ErrNoRows across backend handlers.

Migration includes: jwt.go, local.go, admin_reserve.go, custom_services.go,
discount_campaigns.go, services.go, account.go, customer_relationship.go,
guest.go. Also removes unused test_helpers.go resetTestData function.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/user/customer_relationship.go

@@ -13,6 +13,7 @@ import (
 	"github.com/go-chi/chi/v5"
 
 	"crussell/db"
+	"github.com/jackc/pgx/v5"
 	"crussell/internal/validators"
 )
 
@@ -68,7 +69,7 @@ func GetCustomerRelationshipHandler(w http.ResponseWriter, r *http.Request) {
 		LEFT JOIN payments p ON p.booking_id = b.id
 		WHERE b.user_id = $1
 	`, userID).Scan(&result.TotalSpend, &result.TotalSaved, &result.TotalTips, &result.TotalVisits, &firstVisit, &lastVisit)
-	if err != nil && !errors.Is(err, sql.ErrNoRows) {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		log.Printf("Failed to get customer relationship data for user %s: %v", userID, err)
 	}
 
@@ -113,7 +114,7 @@ func GetCustomerRelationshipHandler(w http.ResponseWriter, r *http.Request) {
 		ORDER BY count DESC
 		LIMIT 5
 	`, userID)
-	if err != nil && !errors.Is(err, sql.ErrNoRows) {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		log.Printf("Failed to get top services for user %s: %v", userID, err)
 	} else {
 		defer rows.Close()