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/admin/discount_campaigns.go

@@ -6,6 +6,7 @@ import (
 	"crussell/mw"
 	"database/sql"
 	"encoding/json"
+	"errors"
 	"log"
 	"net/http"
 	"strconv"
@@ -666,7 +667,7 @@ func GetCampaignStats(w http.ResponseWriter, r *http.Request) {
 		&createdBy,
 	)
 
-	if err == sql.ErrNoRows {
+	if errors.Is(err, pgx.ErrNoRows) {
 		http.Error(w, "Campaign not found", http.StatusNotFound)
 		return
 	}