[feature] custom strings for restock detector

**Version and OS**
[0.47.06](https://github.com/dgtlmoon/changedetection.io/releases/tag/0.47.06) on linux/docker

**Is your feature request related to a problem? Please describe.**

The problem I am facing is that the list of out-of-stock strings in [stock-not-in-stock.js](https://github.com/dgtlmoon/changedetection.io/blob/master/changedetectionio/content_fetchers/res/stock-not-in-stock.js) does not cover this product: https://www.vagabond.com/es/sheila-5635-001-35

¡Pronto estarán en stock!

**Describe the solution you'd like**

I have thought about just opening a PR with my new use case, but maybe what would be better for the long run would be to allow a custom string to be passed to the "Restock & Price Detection" tab when you configure the URL?

**Describe the use-case and give concrete real-world examples**

The use case is using the restock detector feature to parse the fact that size 39 is out of stock for this URL at this moment:

https://www.vagabond.com/es/sheila-5635-001-35

```html

39

```

I could write a CSS selector with the `data-buyable` attribute filtering by the `data-eusize=39` attribute, but this request is about having the restock feature allow me to copy&paste "Pronto estarán en stock" to see that it is not in stock.

I have seen that the page has an ld+json script:

```html

{
"@context": "https://schema.org/",
"@type": "Product",
"name": "SHEILA",
"image": [
"https://cdn.vagabond.com/thumbnails/200/211/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/462/488/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/564/596/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/924/976/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/1264/1336/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/2048/2164/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/200/211/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/462/488/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/564/596/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/924/976/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/1264/1336/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/2048/2164/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/200/211/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/462/488/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/564/596/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/924/976/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/1264/1336/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/2048/2164/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/200/78/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/462/180/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/564/220/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/924/361/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/1264/494/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/2048/800/crop/productimages/pos1/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/200/78/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/462/180/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/564/220/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/924/361/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/1264/494/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/2048/800/crop/productimages/pos2/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/200/78/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/462/180/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/564/220/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/924/361/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/1264/494/crop/productimages/pos3/list/5635-001-35.jpg","https://cdn.vagabond.com/thumbnails/2048/800/crop/productimages/pos3/list/5635-001-35.jpg"],
"description": "Sheila son botas de montar minimalistas con un estilo atemporal. El modelo está confeccionado en cuero marrón oscuro con puntera redondeada, caña ancha y cierre de cremallera lateral. Tacón en bloque mide 38 mm. Cuero marrón oscuro Puntera redondeada Caña ancha Cierre con cremallera Tacones de 38mm Para un cuidado duradero: usa crema para zapatos e impermeabilizante para alargar la vida útil del calzado.",
"sku": "5635-001-35",
"brand": {
"@type": "Thing",
"name": "Vagabond Shoemakers"
},
"offers": {
"@type": "Offer",
"url": "https://www.vagabond.com/es/sheila-5635-001-35/",
"priceCurrency": "EUR",
"price": "220.00",
"itemCondition": "https://schema.org/NewCondition",
"availability": "https://schema.org/InStock",
"seller": {
"@type": "Organization",
"name": "Vagabond International AB"
}
},

"aggregateRating": {
"@type": "AggregateRating",
"ratingValue": 5,
"ratingCount": 3
}
}

```

Unfortunately the item appears as in stock ("availability": "https://schema.org/InStock") because 1 of its variants is, that is there is stock in 2 out of the 7 sizes available. I don't know if this is supposed to be like this in JSON-LD, but it does not make much sense to me since a buyer will usually only buy one variant of the product, so knowing which one is in stock is relevant.

**Additional context**

I am happy to give a go to any changes to the Python side; I am much less comfortable on the JS side of things.

Start with changedetectionio/content_fetchers/res/stock-not-in-stock.js and trace how the Restock & Price Detection tab passes detector settings into the Python side. Reproduce the provided Vagabond HTML and compare current detection with a configurable custom string, including variant-specific behavior. Done means the configured Spanish string is honored without breaking existing stock detection.

Written by the indexing model from the issue text.