Fix UPC barcode: use first 11 digits, not last 11

digits[-11:] was dropping the first digit of 12-digit UPCs.
digits[:11] correctly passes the first 11 digits to the barcode
library, which calculates the matching check digit.

728192558375 now encodes correctly (was 2819255837X before).

disco.py

@@ -204,8 +204,9 @@ def generate_barcode(upc: str, dest_dir: Path) -> Path | None:
     digits = re.sub(r"\D", "", upc)
     if not digits:
         return None
-    # UPC-A needs exactly 11 digits (12th is check digit, auto-calculated)
-    digits = digits[-11:].zfill(11)
+    # UPC-A: pass first 11 digits, library auto-calculates the 12th (check digit)
+    # A full UPC is 12 digits where the 12th is already the check digit
+    digits = digits[:11].zfill(11)
     try:
         upc_cls = barcode.get_barcode_class("upca")
         bc = upc_cls(digits, writer=ImageWriter())