feat: Etiketten-Seitenlayout auf dem A4-Bogen frei waehlbar

- LabelTemplate bekommt columns/rows (optional, leer = automatisch
  passend), plus konfigurierbare Raender/Abstaende - damit ein Etikett
  exakt auf das vorgedruckte Raster eines physischen Etikettenbogens
  (z.B. Avery) gelegt werden kann statt nur automatisch berechnet
- Startposition beim Drucken waehlbar, fuer bereits angebrochene Boegen
  (erste Seite startet ab der gewaehlten Position, Folgeseiten wieder
  von vorne)

ApiCore-Backend/prisma/migrations/20260901173228_label_template_sheet_layout/migration.sql

@@ -0,0 +1,9 @@
+-- Explicit A4 sheet layout per LabelTemplate: columns/rows (nullable, null =
+-- auto-fit) plus configurable page margins and gaps, so a template can be
+-- made to match a specific physical label sheet's exact printed grid.
+ALTER TABLE "LabelTemplate" ADD COLUMN "columns" INTEGER;
+ALTER TABLE "LabelTemplate" ADD COLUMN "rows" INTEGER;
+ALTER TABLE "LabelTemplate" ADD COLUMN "marginTopMm" DOUBLE PRECISION NOT NULL DEFAULT 5;
+ALTER TABLE "LabelTemplate" ADD COLUMN "marginLeftMm" DOUBLE PRECISION NOT NULL DEFAULT 5;
+ALTER TABLE "LabelTemplate" ADD COLUMN "gapXMm" DOUBLE PRECISION NOT NULL DEFAULT 2;
+ALTER TABLE "LabelTemplate" ADD COLUMN "gapYMm" DOUBLE PRECISION NOT NULL DEFAULT 2;

ApiCore-Backend/prisma/schema.prisma

@@ -864,6 +864,17 @@ model LabelTemplate {
   elements  Json     @default("[]")
   widthMm   Float    @default(90)
   heightMm  Float    @default(35)
+  // A4 sheet layout. columns/rows are nullable: null means "auto-fit as many
+  // as fit" (the original behaviour); a real label sheet product (e.g. a
+  // specific Avery SKU) publishes an exact columns/rows/margin/gap grid, so
+  // setting these lets a template's printed positions match the physical,
+  // pre-die-cut sheet instead of just whatever the naive fit computes.
+  columns      Int?
+  rows         Int?
+  marginTopMm  Float @default(5)
+  marginLeftMm Float @default(5)
+  gapXMm       Float @default(2)
+  gapYMm       Float @default(2)
   isDefault Boolean  @default(false)
   active    Boolean  @default(true)
   createdAt DateTime @default(now())