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/Providers/Filament/AdminPanelProvider.php

@@ -11,7 +11,6 @@ use Filament\Pages\Dashboard;
 use Filament\Panel;
 use Filament\PanelProvider;
 use Filament\Support\Colors\Color;
-use Filament\Widgets\AccountWidget;
 use Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse;
 use Illuminate\Cookie\Middleware\EncryptCookies;
 use Illuminate\Foundation\Http\Middleware\PreventRequestForgery;
@@ -39,7 +38,6 @@ class AdminPanelProvider extends PanelProvider
             ])
             ->discoverWidgets(in: app_path('Filament/Widgets'), for: 'App\Filament\Widgets')
             ->widgets([
-                AccountWidget::class,
                 StatsOverviewWidget::class,
             ])
             ->middleware([