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/admin/patch_tests.go

@@ -5,6 +5,7 @@ import (
 	"crussell/internal/validators"
 	"database/sql"
 	"encoding/json"
+	"log/slog"
 	"net/http"
 	"strconv"
 
@@ -73,7 +74,7 @@ func GetPatchTests(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	json.NewEncoder(w).Encode(patchTests)
+	_ = json.NewEncoder(w).Encode(patchTests)
 }
 
 // CreatePatchTest handles POST /api/admin/patch-tests
@@ -100,7 +101,11 @@ func CreatePatchTest(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)
+    }
+}()
 
 	var id string
 	err = tx.QueryRow(r.Context(), query, req.Name, req.Description, req.NoticeDurationHours, req.ExpiryMonths, req.ServiceIDs).Scan(&id)
@@ -115,7 +120,7 @@ func CreatePatchTest(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.WriteHeader(http.StatusCreated)
-	json.NewEncoder(w).Encode(map[string]string{"id": id})
+	_ = json.NewEncoder(w).Encode(map[string]string{"id": id})
 }
 
 // UpdatePatchTest handles PUT /api/admin/patch-tests/{id}
@@ -176,7 +181,11 @@ func UpdatePatchTest(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)
+    }
+}()
 
 	_, err = tx.Exec(r.Context(), query, args...)
 	if err != nil {
@@ -205,7 +214,11 @@ func DeletePatchTest(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)
+    }
+}()
 
 	_, err = tx.Exec(r.Context(), "DELETE FROM patch_tests WHERE id = $1", id)
 	if err != nil {