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/services/services.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crussell/auth"
 	"crussell/db"
+	"github.com/jackc/pgx/v5"
 	"crussell/internal/validators"
 	"crussell/mw"
 	"database/sql"
@@ -407,7 +408,7 @@ func ServicesEligibleForUserHandler(w http.ResponseWriter, r *http.Request) {
 	// Get user's date of birth
 	var dob time.Time
 	err := db.DB.QueryRow(r.Context(), `SELECT date_of_birth FROM users WHERE id = $1`, userID).Scan(&dob)
-	if errors.Is(err, sql.ErrNoRows) {
+	if errors.Is(err, pgx.ErrNoRows) {
 		http.Error(w, "user not found", http.StatusNotFound)
 		return
 	}