feat(upload): warn on duplicate filename with link to existing document

- storeDocument now returns StoreResult(document, isNew) to distinguish
  new uploads from updates to existing documents
- QuickUploadResult gains an `updated` list alongside `created`
- Frontend shows an amber warning with a "View document" link for duplicates
  instead of silently re-uploading and leaving the user confused

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

frontend/messages/es.json

@@ -271,6 +271,8 @@
 	"upload_drop_hint": "Soltar archivos o hacer clic para seleccionar",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
 	"upload_success": "{count} documento(s) creado(s)",
+	"upload_duplicate": "{filename} ya existe —",
+	"upload_duplicate_link": "Ver documento",
 	"upload_invalid_type": "{filename}: formato de archivo no admitido",
 	"upload_error": "Error al subir {filename}"
 }