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/auth/local.go

@@ -4,12 +4,12 @@ import (
 	"context"
 	"crussell/auth"
 	"crussell/db"
+	"github.com/jackc/pgx/v5"
 	"crussell/internal/dav"
 	"crussell/internal/validators"
 	"crussell/internal/zxcvbnjs"
 	"crussell/mw"
 	"crypto/rand"
-	"database/sql"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -520,7 +520,7 @@ func GenerateVerificationCodeHandler(w http.ResponseWriter, r *http.Request) {
 		"SELECT id FROM users WHERE LOWER(email) = $1", email,
 	).Scan(&userID)
 	if err != nil {
-		if errors.Is(err, sql.ErrNoRows) {
+		if errors.Is(err, pgx.ErrNoRows) {
 			w.Header().Set("Content-Type", "application/json")
 			json.NewEncoder(w).Encode(VerificationResponse{Success: true, Message: "If the email exists, a verification code will be sent"})
 			return
@@ -576,7 +576,7 @@ func VerifyCodeHandler(w http.ResponseWriter, r *http.Request) {
 		code,
 	).Scan(&userID, &purpose, &expiresAt)
 	if err != nil {
-		if errors.Is(err, sql.ErrNoRows) {
+		if errors.Is(err, pgx.ErrNoRows) {
 			// Check if code exists but was already used or expired
 			var checkUsedAt *time.Time
 			checkErr := db.DB.QueryRow(r.Context(),