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/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