v1.8.3 — Fix Overpass CORS rejection

Bugfix

POI markers stopped appearing because the main `overpass-api.de` instance recently tightened its CORS policy and now rejects POST preflights from browsers with HTTP 406:

```
Origin http:// is not allowed by Access-Control-Allow-Origin
Status code: 406
Fetch API cannot load https://overpass-api.de/api/interpreter
```

Two changes fix this:

1. Switch from POST to GET. The query is small enough (~500 chars) to fit in a URL. GET requests are "simple" CORS requests and don't require a preflight, so the new restrictive policy doesn't apply.

2. Automatic mirror fallback. If the primary endpoint fails or returns nothing, the card transparently retries with two community mirrors (`overpass.kumi.systems`, `overpass.private.coffee`). Each failure is logged with the endpoint URL so users can see which one rejected.

Action after update

If you toggled POI on a ride before this update and saw nothing, your browser may still hold an empty cached result. To clear:

```js
Object.keys(localStorage).filter(k=>k.startsWith("eb-poi-")).forEach(k=>localStorage.removeItem(k))
```

(Run that in the Web-Inspektor console.)

Installation

Update via HACS, restart Home Assistant, hard-refresh the browser (Cmd+Shift+R / Ctrl+Shift+R).