Use UPC (not SKU) for barcode generation

UPC-A barcodes should encode the Universal Product Code, not the
internal store SKU. The UPCs are already 12-digit numbers that match
the barcodes on the physical product packaging.

disco.py

@@ -199,9 +199,9 @@ def make_placeholder(dest: Path, text: str = "No Image") -> Path:
     return dest
 
 
-def generate_barcode(sku: str, dest_dir: Path) -> Path | None:
-    """Generate a UPC-A barcode PNG from a SKU. Returns path to the .png file."""
-    digits = re.sub(r"\D", "", sku)
+def generate_barcode(upc: str, dest_dir: Path) -> Path | None:
+    """Generate a UPC-A barcode PNG from a UPC number. Returns path to the .png file."""
+    digits = re.sub(r"\D", "", upc)
     if not digits:
         return None
     # UPC-A needs exactly 11 digits (12th is check digit, auto-calculated)
@@ -210,7 +210,7 @@ def generate_barcode(sku: str, dest_dir: Path) -> Path | None:
         upc_cls = barcode.get_barcode_class("upca")
         bc = upc_cls(digits, writer=ImageWriter())
         # barcode lib appends .png automatically
-        out = dest_dir / f"barcode_{sku}"
+        out = dest_dir / f"barcode_{upc}"
         saved = bc.save(
             str(out),
             options={
@@ -223,7 +223,7 @@ def generate_barcode(sku: str, dest_dir: Path) -> Path | None:
         )
         return Path(saved)
     except Exception as e:
-        print(f"   ⚠ Barcode generation failed for {sku}: {e}")
+        print(f"   ⚠ Barcode generation failed for {upc}: {e}")
         return None
 
 # ---------------------------------------------------------------------------
@@ -291,8 +291,8 @@ def generate_catalog_pdf(products: list[dict]) -> Path | None:
                 IMAGES_DIR / f"product_{i}_{sku}_placeholder.png", title[:30]
             )
 
-        # Generate barcode
-        bc_path = generate_barcode(sku, BARCODES_DIR)
+        # Generate barcode from UPC (not SKU)
+        bc_path = generate_barcode(upc, BARCODES_DIR)
 
         # Escape LaTeX special characters in text fields
         safe_title = (