Update golangci-lint to v2.12.2 with canonical config

Bump golangci-lint from v2.10.1 to v2.12.2 in the Dockerfile lint
stage (tag+digest pin) and script/bootstrap release-archive pins
(linux amd64/arm64 sha256s). Replace .golangci.yml with the canonical
v2-layout config so linter settings (lll 88, funlen 80/50, cyclop 15,
dupl 100) actually apply.

Fix all findings surfaced by the new linter and config:

- noctx: use httptest.NewRequestWithContext in all tests
- gosec G710/G703: route app redirects through a path-escaping
  redirectToApp helper; annotate internal log path usage
- goconst: introduce shared constants for template/JSON keys and
  repeated test literals
- lll: wrap lines to the 88-column limit
- dupl: extract shared helpers (generic findAllByAppID in models,
  deleteAppResource in handlers, parsePush in webhook payloads,
  table-driven/helper-based test dedup)
- nolintlint: drop nolint directives made obsolete by the new limits

Record the change in TODO.md; make check is green.

internal/models/label.go

@@ -93,6 +93,10 @@ func FindLabel(
 	return label, nil
 }
 
+func (l *Label) scanDest() []any {
+	return []any{&l.ID, &l.AppID, &l.Key, &l.Value}
+}
+
 // FindLabelsByAppID finds all labels for an app.
 func FindLabelsByAppID(
 	ctx context.Context,
@@ -103,27 +107,7 @@ func FindLabelsByAppID(
 		SELECT id, app_id, key, value FROM app_labels
 		WHERE app_id = ? ORDER BY key`
 
-	rows, err := db.Query(ctx, query, appID)
-	if err != nil {
-		return nil, fmt.Errorf("querying labels by app: %w", err)
-	}
-
-	defer func() { _ = rows.Close() }()
-
-	var labels []*Label
-
-	for rows.Next() {
-		label := NewLabel(db)
-
-		scanErr := rows.Scan(&label.ID, &label.AppID, &label.Key, &label.Value)
-		if scanErr != nil {
-			return nil, scanErr
-		}
-
-		labels = append(labels, label)
-	}
-
-	return labels, rows.Err()
+	return findAllByAppID(ctx, db, query, appID, "labels", NewLabel)
 }
 
 // DeleteLabelsByAppID deletes all labels for an app.