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
 		}