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/services/services.go

@@ -10,6 +10,7 @@ import (
 	"encoding/json"
 	"errors"
 	"github.com/jackc/pgx/v5"
+	"log/slog"
 	"net/http"
 	"time"
 
@@ -64,7 +65,11 @@ func ToggleService(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "Internal server error", http.StatusInternalServerError)
 		return
 	}
-	defer tx.Rollback(r.Context())
+	defer func() {
+    if err := tx.Rollback(r.Context()); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	query := "UPDATE services SET is_active = NOT is_active WHERE id = $1"
 	result, err := tx.Exec(r.Context(), query, serviceID)
@@ -84,7 +89,7 @@ func ToggleService(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.WriteHeader(http.StatusOK)
-	json.NewEncoder(w).Encode(map[string]any{
+	_ = json.NewEncoder(w).Encode(map[string]any{
 		"message": "Service toggled successfully",
 		"id":      serviceID,
 	})
@@ -137,7 +142,11 @@ func CreateServiceHandler(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "Internal server error", http.StatusInternalServerError)
 		return
 	}
-	defer tx.Rollback(r.Context())
+	defer func() {
+    if err := tx.Rollback(r.Context()); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	query := `
 		INSERT INTO services (
@@ -227,7 +236,11 @@ func DeleteServiceHandler(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "Internal server error", http.StatusInternalServerError)
 		return
 	}
-	defer tx.Rollback(r.Context())
+	defer func() {
+    if err := tx.Rollback(r.Context()); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	query := "UPDATE services SET is_active = FALSE WHERE id = $1"
 	result, err := tx.Exec(r.Context(), query, serviceID)
@@ -247,7 +260,7 @@ func DeleteServiceHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.WriteHeader(http.StatusOK)
-	json.NewEncoder(w).Encode(map[string]any{
+	_ = json.NewEncoder(w).Encode(map[string]any{
 		"message": "Service deleted successfully",
 		"id":      serviceID,
 	})