F-Droid downloads badge

### 📋 Description

F-Droid

Example for package [com.github.catfriend1.syncthingandroid](https://f-droid.org/packages/com.github.catfriend1.syncthingandroid/):

Example for package [io.github.kitswas.virtualgamepadmobile](https://f-droid.org/packages/io.github.kitswas.virtualgamepadmobile/):

These are dynamic badges. The larger numbers are not easily readable.

Also, the dynamic badge shows resource not found if there are zero downloads.

### 🔗 Data

Documentation: https://github.com/kitswas/fdroid-metrics-dashboard/?tab=readme-ov-file#badges-via-shieldsio

Example of data file:
https://github.com/kitswas/fdroid-metrics-dashboard/blob/main/processed/monthly/com.termux.json

```json
{
"package_id": "com.termux",
"total_downloads": 505603,
"api_hits": 35908,
"versions": 7,
"search_count": 241
}
```

### 🎤 Motivation

All other major distribution centres have a download count.

Time to add F-Droid to the list.

Start with the Shields documentation and the linked F-Droid metrics dashboard, especially the “badges via shields.io” section and the sample monthly JSON file. Confirm how the F-Droid endpoint and query should be supported, including zero-download responses, then verify that the resulting badge renders readable counts without a resource-not-found error.

Written by the indexing model from the issue text.