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/custom_services.go

@@ -6,11 +6,13 @@ import (
 	"crussell/mw"
 	"database/sql"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"strconv"
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/jackc/pgx/v5"
 )
 
 type CustomService struct {
@@ -268,7 +270,7 @@ func GetCustomService(w http.ResponseWriter, r *http.Request) {
 		SELECT id, name, description, price, duration_minutes, minimum_age_required, notes, created_at, created_by, usage_count, last_used_at
 		FROM custom_services WHERE id = $1
 	`, id).Scan(&cs.ID, &cs.Name, &desc, &cs.Price, &cs.DurationMinutes, &cs.MinimumAgeRequired, &notes, &cs.CreatedAt, &createdBy, &cs.UsageCount, &lastUsedAt)
-	if err == sql.ErrNoRows {
+	if errors.Is(err, pgx.ErrNoRows) {
 		http.Error(w, "Custom service not found", http.StatusNotFound)
 		return
 	}
@@ -386,7 +388,7 @@ func PromoteCustomService(w http.ResponseWriter, r *http.Request) {
 		SELECT name, description, price, duration_minutes, minimum_age_required, notes, created_by
 		FROM custom_services WHERE id = $1
 	`, id).Scan(&name, &desc, &price, &durationMinutes, &minimumAgeRequired, &notes, &createdBy)
-	if err == sql.ErrNoRows {
+	if errors.Is(err, pgx.ErrNoRows) {
 		http.Error(w, "Custom service not found", http.StatusNotFound)
 		return
 	}
@@ -454,7 +456,7 @@ func DeleteCustomService(w http.ResponseWriter, r *http.Request) {
 
 	var usageCount int
 	err := db.DB.QueryRow(r.Context(), `SELECT usage_count FROM custom_services WHERE id = $1`, id).Scan(&usageCount)
-	if err == sql.ErrNoRows {
+	if errors.Is(err, pgx.ErrNoRows) {
 		http.Error(w, "Custom service not found", http.StatusNotFound)
 		return
 	}

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
 	}

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 {