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/bookings/reserve.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"log/slog"
 	"net/http"
 	"strings"
 	"time"
@@ -179,7 +180,11 @@ func ReserveSlotHandler(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)
+    }
+}()
 
 		// Check booking overlap inside transaction (TOCTOU fix)
 		// pending_release is excluded — those bookings are evicted at creation time.
@@ -249,7 +254,11 @@ func ReserveSlotHandler(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)
+    }
+}()
 
 		// Check anon rate cap inside transaction
 		tenMinutesAgo := clock.Now().Add(-10 * time.Minute)
@@ -333,5 +342,5 @@ func ReserveSlotHandler(w http.ResponseWriter, r *http.Request) {
 
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusCreated)
-	json.NewEncoder(w).Encode(response)
+	_ = json.NewEncoder(w).Encode(response)
 }