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/test_helpers.go

@@ -10,21 +10,12 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"strings"
-	"testing"
 
-	"crussell/db"
 	"crussell/mw"
-	"crussell/testutils/testdb"
 
 	"github.com/go-chi/chi/v5"
 )
 
-// resetTestData truncates tables to clean up data between tests
-func resetTestData(t *testing.T) {
-	t.Helper()
-	testdb.TruncateTables(t, db.DB)
-}
-
 // makeAdminRequest creates a request with admin context
 // Note: Using 12-char IDs to match CHAR(12) columns in schema (e.g., created_by)
 func makeAdminRequest(handler http.Handler, method, path string, body interface{}) *httptest.ResponseRecorder {