feat(labels): cut marks in the sheet margin for trimming with a paper cutter

Label sheets printed on plain A4 are cut with a lever paper cutter. Each
template can now print small hairline cut marks in the sheet margin: one
at both ends of every cut line (left/right edge of each column, top/bottom
edge of each row), 1 mm away from the label and never closer than 2 mm to
the paper edge, so they print on normal printers and never touch a label.
Labels without a gap share one mark. Marks are drawn on every sheet for
the full grid, since the cutter cuts across the whole sheet.

On by default (new column cropMarks, default true); switch it off in the
template designer for pre-die-cut label sheets.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Backend/prisma/migrations/20260928040804_label_template_crop_marks/migration.sql

@@ -0,0 +1,3 @@
+-- AlterTable
+ALTER TABLE "LabelTemplate" ADD COLUMN     "cropMarks" BOOLEAN NOT NULL DEFAULT true;
+

ApiCore-Backend/prisma/schema.prisma

@@ -929,6 +929,10 @@ model LabelTemplate {
   marginLeftMm Float    @default(5)
   gapXMm       Float    @default(2)
   gapYMm       Float    @default(2)
+  // Kleine Schnittmarken im Blattrand, eine an jedem Ende jeder
+  // Schnittlinie - zum Anlegen am Papierschneider. Aus bei vorgestanzten
+  // Etikettenbögen, die nicht geschnitten werden.
+  cropMarks    Boolean  @default(true)
   isDefault    Boolean  @default(false)
   active       Boolean  @default(true)
   createdAt    DateTime @default(now())