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/SignalContext.php

@@ -1,24 +0,0 @@
-<?php
-
-namespace App\Services\Prediction\Signals;
-
-use App\Enums\FuelType;
-
-/**
- * Inputs required to evaluate a prediction signal. Individual signals may
- * ignore fields they don't need — for example OilSignal doesn't use fuelType,
- * RegionalMomentumSignal requires lat/lng to be non-null.
- */
-final readonly class SignalContext
-{
-    public function __construct(
-        public FuelType $fuelType,
-        public ?float $lat = null,
-        public ?float $lng = null,
-    ) {}
-
-    public function hasCoordinates(): bool
-    {
-        return $this->lat !== null && $this->lng !== null;
-    }
-}