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/bookings/admin_reserve.go

@@ -3,9 +3,9 @@ package bookings
 import (
 	"context"
 	"crussell/db"
+	"github.com/jackc/pgx/v5"
 	"crussell/handlers/scheduling"
 	"crussell/mw"
-	"database/sql"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -111,7 +111,7 @@ func AdminReserveSlotHandler(w http.ResponseWriter, r *http.Request) {
 	weekday := int((localStart.Weekday() + 6) % 7)
 	var closeStr string
 	if err := db.DB.QueryRow(r.Context(), `SELECT end_time::text FROM working_hours WHERE weekday = $1`, weekday).Scan(&closeStr); err != nil {
-		if errors.Is(err, sql.ErrNoRows) {
+		if errors.Is(err, pgx.ErrNoRows) {
 			http.Error(w, "Not open on this day", http.StatusBadRequest)
 			return
 		}