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/internal/dav/shared.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crussell/clock"
 	"fmt"
+	"log/slog"
 	"strings"
 	"time"
 
@@ -181,7 +182,11 @@ func (s *BaseService) CreateContact(addressBookID int, userID string, input Cont
 	if err != nil {
 		return err
 	}
-	defer tx.Rollback(context.Background())
+	defer func() {
+    if err := tx.Rollback(context.Background()); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	query := `
 		INSERT INTO dav_cards (addressbookid, uri, carddata, lastmodified, etag, size)
@@ -211,7 +216,11 @@ func (s *BaseService) UpdateContact(addressBookID int, uri string, input Contact
 	if err != nil {
 		return err
 	}
-	defer tx.Rollback(context.Background())
+	defer func() {
+    if err := tx.Rollback(context.Background()); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	query := `
 		UPDATE dav_cards
@@ -232,7 +241,11 @@ func (s *BaseService) DeleteContact(addressBookID int, uri string) error {
 	if err != nil {
 		return err
 	}
-	defer tx.Rollback(context.Background())
+	defer func() {
+    if err := tx.Rollback(context.Background()); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	query := `DELETE FROM dav_cards WHERE addressbookid = $1 AND uri = $2`
 	_, err = tx.Exec(context.Background(), query, addressBookID, uri)
@@ -253,7 +266,11 @@ func (s *BaseService) CreateEvent(calendarID int, input EventInput) error {
 	if err != nil {
 		return err
 	}
-	defer tx.Rollback(context.Background())
+	defer func() {
+    if err := tx.Rollback(context.Background()); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	query := `
 		INSERT INTO dav_calendarobjects
@@ -288,7 +305,11 @@ func (s *BaseService) UpdateEvent(calendarID int, uid string, input EventInput)
 	if err != nil {
 		return err
 	}
-	defer tx.Rollback(context.Background())
+	defer func() {
+    if err := tx.Rollback(context.Background()); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	query := `
 		UPDATE dav_calendarobjects
@@ -314,7 +335,11 @@ func (s *BaseService) DeleteEvent(calendarID int, uid string) error {
 	if err != nil {
 		return err
 	}
-	defer tx.Rollback(context.Background())
+	defer func() {
+    if err := tx.Rollback(context.Background()); err != nil {
+        slog.Error("failed to rollback transaction", "err", err)
+    }
+}()
 
 	query := `DELETE FROM dav_calendarobjects WHERE calendarid = $1 AND uid = $2`
 	_, err = tx.Exec(context.Background(), query, calendarID, uid)

backend/internal/jobs/scheduler.go

@@ -40,7 +40,9 @@ func init() {
 	hostname = h
 
 	var buf [12]byte
-	rand.Read(buf[:])
+	if _, err := rand.Read(buf[:]); err != nil {
+		panic("crypto/rand.Read failed: " + err.Error())
+	}
 	b64 := base64.StdEncoding.EncodeToString(buf[:])
 	b64 = strings.NewReplacer("+", "", "/", "").Replace(b64)
 	randomPrefix = b64[0:10]
@@ -53,7 +55,6 @@ func Hostname() string { return hostname }
 // Shorthand references to logutil constants (avoids package-qualified noise).
 var (
 	colorReset       = logutil.Reset
-	colorBold        = logutil.Bold
 	colorDim         = logutil.Dim
 	colorCyan        = logutil.Cyan
 	colorGreen       = logutil.Green