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/auth/jwt.go

@@ -3,12 +3,12 @@ package auth
 import (
 	"context"
 	"crypto/rand"
-	"database/sql"
 	"errors"
 	"fmt"
 	"time"
 
 	"crussell/db"
+	"github.com/jackc/pgx/v5"
 
 	"github.com/go-chi/jwtauth/v5"
 )
@@ -196,7 +196,7 @@ func VerifyRefreshToken(ctx context.Context, tokenString string) (userID string,
 
 	err = db.DB.QueryRow(ctx, query, tokenString).Scan(&userID, &role)
 	if err != nil {
-		if errors.Is(err, sql.ErrNoRows) {
+		if errors.Is(err, pgx.ErrNoRows) {
 			return "", "", fmt.Errorf("invalid or expired refresh token")
 		}
 		return "", "", fmt.Errorf("failed to verify refresh token: %w", err)