fix: resolve golangci-lint violations (errcheck, unused, gosimple, ineffassign)

errcheck: add proper error handling with slog.Error for tx.Rollback, key generation, and s3/dav operations. Add nolint comments for intentionally discarded DB scan errors and HTTP write errors.
unused: remove dead code (svcRow type, processImage, nonDepositPaymentType, generateSecureCode, colorBold, nGreen, nRed)
gosimple S1021: merge var declaration with assignment in manage.go
ineffassign: remove dead assignments in settings.go, till.go, images.go

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/payments/service.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"log"
+	"log/slog"
 	"strconv"
 	"strings"
 	"time"
@@ -468,7 +469,11 @@ func (s *PaymentService) DeletePaymentMethod(ctx context.Context, cardID, userID
 		log.Printf("Failed to begin transaction: %v", err)
 		return err
 	}
-	defer tx.Rollback(ctx)
+	defer func() {
+    if err := tx.Rollback(ctx); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	retainedUntil := clock.Now().Add(7 * 365 * 24 * time.Hour)
 	_, err = tx.Exec(ctx, `