fix: guard col I/J/K access for short rows in MTZ Excel

MTZ changed their Excel format — some rows have fewer than 10 columns,
causing IndexError on row[9] (brand, col J). Use conditional indexing for
cols I (8), J (9), K (10) so short rows parse with empty brand/type/min_box
instead of crashing on startup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

main.py

@@ -90,10 +90,10 @@ def download_and_parse():
             skipped += 1
             continue
 
-        brand_raw = row[9]  # col J
+        type_raw = row[8] if len(row) > 8 else None    # col I
+        brand_raw = row[9] if len(row) > 9 else None   # col J
         brand = str(brand_raw) if brand_raw is not None else ""
-        min_box_raw = row[10]  # col K
-        type_raw = row[8]  # col I
+        min_box_raw = row[10] if len(row) > 10 else None  # col K
         product_type = re.sub(r'^\d+\.', '', str(type_raw)).strip() if type_raw is not None else ""
         description = re.sub(r'\s*\(\d+\).*$', '', raw_desc).strip()
         parsed.append(