debug: print response content-type and body on non-Excel download

Helps identify what the server is actually returning when the
download URL doesn't serve the xlsx file.

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

scripts/sync-elo.py

@@ -32,7 +32,15 @@ def main():
     print(f"Downloading ELO sheet…", flush=True)
     resp = requests.get(DOWNLOAD_URL, timeout=120, headers={"User-Agent": "Mozilla/5.0"})
     resp.raise_for_status()
-    print(f"Downloaded {len(resp.content):,} bytes", flush=True)
+
+    content_type = resp.headers.get("Content-Type", "")
+    print(f"Downloaded {len(resp.content):,} bytes  Content-Type: {content_type}", flush=True)
+
+    # Diagnose non-Excel responses before attempting to parse
+    if "html" in content_type.lower() or not resp.content.startswith(b"PK"):
+        print("ERROR: Response is not an Excel/ZIP file. First 500 chars:", flush=True)
+        print(resp.content[:500].decode("utf-8", errors="replace"), flush=True)
+        sys.exit(1)
 
     wb = openpyxl.load_workbook(BytesIO(resp.content), read_only=True, data_only=True)
     ws = wb.active