fix: ventas page only shows CMXC records, remove duplicate column

- _query_rows now always filters by PREFIJO='CMXC' (unless user specifies another prefix in the factura field)
- Removed redundant first column IDRECEP from ventas table (factura_key and factura were identical)
- Renamed header to just Factura

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

app/routes/ventas.py

@@ -43,8 +43,9 @@ def _query_rows(cfg, query_text, factura, fecha_inicio, fecha_fin):
     if not ok2:
         return None, err
 
-    if prefix and rows and "PREFIJO" in rows[0]:
-        rows = [r for r in rows if str(r.get("PREFIJO") or "").strip().upper() == prefix]
+    if rows and "PREFIJO" in rows[0]:
+        filter_prefix = prefix if prefix else "CMXC"
+        rows = [r for r in rows if str(r.get("PREFIJO") or "").strip().upper() == filter_prefix]
 
     return rows, None