docs: add comprehensive API reference documentation

Adds complete API reference (api-reference.md) covering all endpoints: /api/stations (nearby search with postcode/lat+lng), /api/stats/searches (aggregated search stats), /api/prediction (7-day price forecast with multi-signal breakdown), and auth routes (register/login/logout/me). Includes request/response examples, error shapes, fuel type aliases, and signal structure details. Also removes unused AccountWidget from admin panel, disables BrentPriceChartWidget discovery, and adds searches stat to StatsOverviewWidget.

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

app/Filament/Widgets/BrentPriceChartWidget.php

@@ -11,6 +11,8 @@ class BrentPriceChartWidget extends ChartWidget
 
     protected ?string $pollingInterval = null;
 
+    protected static bool $isDiscovered = false;
+
     protected function getData(): array
     {
         $prices = BrentPrice::orderBy('date')