chore: retire legacy oil prediction pipeline

Removes everything that was made redundant by the new forecasting
stack. Per docs/superpowers/specs/2026-05-01-prediction-rebuild-design.md,
this was the cleanup planned at the end of Phase 4.

Deleted services and code:
- App\Services\Prediction\Signals\* (the old six-signal aggregator —
  trend, supermarket, day-of-week, brand-behaviour, stickiness,
  regional-momentum, oil — replaced by RidgeRegressionModel).
- App\Services\NationalFuelPredictionService (the post-Phase-4 thin
  shim; StationSearchService now depends on WeeklyForecastService
  directly, set up in the previous commit).
- App\Services\LlmPrediction\* (AbstractLlmPredictionProvider plus
  the four provider implementations — Anthropic, OpenAI, Gemini, and
  the OilPredictionProvider router. Replaced by LlmOverlayService).
- App\Services\BrentPricePredictor and App\Services\Ewma. The Ewma
  helper had no callers left after BrentPricePredictor went.
- App\Models\PricePrediction and its factory.
- App\Console\Commands\PredictOilPrices (the oil:predict command).
- App\Filament\Resources\OilPredictionResource and its Pages.

Schema and dashboard:
- Drop the price_predictions table via a new migration.
- Repoint the Filament StatsOverviewWidget tile from PricePrediction
  to WeeklyForecast so the dashboard reflects the new pipeline.
- Remove the OilPredictionProvider binding from AppServiceProvider.

Test cleanup:
- Delete tests for every retired service.
- Update StatsOverviewWidgetTest to seed weekly_forecasts instead of
  price_predictions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/Services/Prediction/Signals/DayOfWeekSignal.php

@@ -1,80 +0,0 @@
-<?php
-
-namespace App\Services\Prediction\Signals;
-
-use Illuminate\Support\Facades\DB;
-
-final class DayOfWeekSignal extends AbstractSignal
-{
-    private const int MIN_DAYS = 21;
-
-    public function compute(SignalContext $context): array
-    {
-        $dowExpr = DbDialect::dayOfWeekExpr('price_effective_at');
-
-        $rows = DB::table('station_prices')
-            ->where('fuel_type', $context->fuelType->value)
-            ->where('price_effective_at', '>=', now()->subDays(90))
-            ->selectRaw("{$dowExpr} as dow, DATE(price_effective_at) as day, AVG(price_pence) as avg_price")
-            ->groupBy('dow', 'day')
-            ->get();
-
-        $uniqueDays = $rows->pluck('day')->unique()->count();
-
-        if ($uniqueDays < self::MIN_DAYS) {
-            return $this->disabledSignal("Insufficient history for day-of-week pattern ({$uniqueDays} days, need ".self::MIN_DAYS.')');
-        }
-
-        $dowAverages = $rows->groupBy('dow')->map(fn ($g) => $g->avg('avg_price'));
-        $weekAvg = $dowAverages->avg();
-        $todayDow = (int) now()->format('w') + 1; // PHP 0=Sun → MySQL 1=Sun
-        $todayAvg = $dowAverages->get($todayDow, $weekAvg);
-        $cheapestDow = $dowAverages->keys()->sortBy(fn ($k) => $dowAverages[$k])->first();
-        $dayNames = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
-        $todayName = $dayNames[($todayDow - 1) % 7] ?? 'Today';
-        $tomorrowName = $dayNames[$todayDow % 7] ?? 'Tomorrow';
-
-        $todayDeltaPence = round(($todayAvg - $weekAvg) / 100, 1);
-        $tomorrowDeltaPence = round(($dowAverages->get(($todayDow % 7) + 1, $weekAvg) - $todayAvg) / 100, 1);
-
-        $direction = match (true) {
-            ($todayAvg - $weekAvg) / 100 >= 1.5 => 'up',
-            ($weekAvg - $todayAvg) / 100 >= 1.5 => 'down',
-            default => 'stable',
-        };
-
-        $score = $direction === 'stable' ? 0.0 : ($direction === 'up' ? 1.0 : -1.0);
-
-        $parts = [];
-        $parts[] = abs($todayDeltaPence) < 0.1
-            ? "Today ({$todayName}) is typically in line with the weekly average."
-            : sprintf(
-                'Today (%s) is typically %sp %s the weekly average.',
-                $todayName,
-                number_format(abs($todayDeltaPence), 1),
-                $todayDeltaPence > 0 ? 'above' : 'below',
-            );
-
-        $parts[] = abs($tomorrowDeltaPence) < 0.1
-            ? "Tomorrow ({$tomorrowName}) is typically the same."
-            : sprintf(
-                'Tomorrow (%s) is typically %sp %s.',
-                $tomorrowName,
-                number_format(abs($tomorrowDeltaPence), 1),
-                $tomorrowDeltaPence < 0 ? 'cheaper' : 'pricier',
-            );
-
-        if ($cheapestDow === $todayDow) {
-            $parts[] = 'Today is historically the cheapest day of the week.';
-        }
-
-        return [
-            'score' => $score,
-            'confidence' => min(1.0, $uniqueDays / 90),
-            'direction' => $direction,
-            'detail' => implode(' ', $parts),
-            'data_points' => $uniqueDays,
-            'enabled' => true,
-        ];
-    }
-}