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/account.go

@@ -3,6 +3,7 @@ package user
 import (
 	"context"
 	"database/sql"
+	"errors"
 	"fmt"
 	"log"
 	"net/http"
@@ -13,6 +14,7 @@ import (
 	"crussell/internal/dav"
 	"crussell/internal/s3"
 	"crussell/mw"
+	"github.com/jackc/pgx/v5"
 )
 
 // DELETE /api/user/account
@@ -28,7 +30,7 @@ func DeleteAccountHandler(w http.ResponseWriter, r *http.Request) {
 	err := db.DB.QueryRow(r.Context(), `SELECT account_role, profile_pic_url FROM users WHERE id = $1`, userID).
 		Scan(&accountRole, &profilePicURL)
 	if err != nil {
-		if err == sql.ErrNoRows {
+		if errors.Is(err, pgx.ErrNoRows) {
 			http.Error(w, "user not found", http.StatusNotFound)
 			return
 		}

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()

backend/handlers/user/guest.go

@@ -1,7 +1,6 @@
 package user
 
 import (
-	"database/sql"
 	"encoding/json"
 	"errors"
 	"log"
@@ -12,6 +11,7 @@ import (
 	"crussell/db"
 	"crussell/handlers/auth"
 	"crussell/internal/validators"
+	"github.com/jackc/pgx/v5"
 )
 
 type CreateGuestUserRequest struct {
@@ -172,7 +172,7 @@ func CheckEmailHandler(w http.ResponseWriter, r *http.Request) {
 			s := "check"
 			suggestion = &s
 		}
-	} else if !errors.Is(err, sql.ErrNoRows) {
+	} else if !errors.Is(err, pgx.ErrNoRows) {
 		log.Printf("Failed to check email: %v", err)
 		http.Error(w, "database error", http.StatusInternalServerError)
 		return