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/user/account.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"log"
+	"log/slog"
 	"net/http"
 	"os"
 
@@ -103,7 +104,11 @@ func DeleteAccountHandler(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, "server error", http.StatusInternalServerError)
 			return
 		}
-		defer tx.Rollback(ctx)
+		defer func() {
+    if err := tx.Rollback(ctx); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 		_, err = tx.Exec(ctx, `SELECT delete_guest_user($1)`, userID)
 		if err != nil {
@@ -124,7 +129,11 @@ func DeleteAccountHandler(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, "server error", http.StatusInternalServerError)
 			return
 		}
-		defer tx.Rollback(ctx)
+		defer func() {
+    if err := tx.Rollback(ctx); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 		_, err = tx.Exec(ctx, `SELECT anonymize_user($1)`, userID)
 		if err != nil {