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/auth/jwt.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"log"
+	"log/slog"
 	"time"
 
 	"crussell/clock"
@@ -47,7 +48,11 @@ func RevokeJTI(ctx context.Context, jti string, expiresAt time.Time) {
 		log.Printf("WARN: Failed to begin transaction for JTI revocation: %v", err)
 		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,
 		`INSERT INTO revoked_jtis (jti, expires_at) VALUES ($1, $2)
@@ -92,7 +97,11 @@ func CleanupRevokedJTIs(ctx context.Context) (int, error) {
 		log.Printf("WARN: Failed to begin transaction for JTI cleanup: %v", err)
 		return 0, err
 	}
-	defer tx.Rollback(ctx)
+	defer func() {
+    if err := tx.Rollback(ctx); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	tag, err := tx.Exec(ctx,
 		`DELETE FROM revoked_jtis WHERE expires_at < NOW()`)
@@ -198,7 +207,11 @@ func GenerateRefreshToken(ctx context.Context, userID string, role string) (stri
 	if err != nil {
 		return "", fmt.Errorf("failed to begin transaction: %w", err)
 	}
-	defer tx.Rollback(ctx)
+	defer func() {
+    if err := tx.Rollback(ctx); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	var tokenID int64
 	err = tx.QueryRow(ctx, query, userID, token, role).Scan(&tokenID)
@@ -227,7 +240,11 @@ func VerifyRefreshToken(ctx context.Context, tokenString string) (userID string,
 	if err != nil {
 		return "", "", fmt.Errorf("failed to begin transaction: %w", err)
 	}
-	defer tx.Rollback(ctx)
+	defer func() {
+    if err := tx.Rollback(ctx); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	err = tx.QueryRow(ctx, query, tokenString).Scan(&userID, &role)
 	if err != nil {