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>

database/factories/PricePredictionFactory.php

@@ -1,34 +0,0 @@
-<?php
-
-namespace Database\Factories;
-
-use App\Enums\PredictionSource;
-use App\Enums\TrendDirection;
-use App\Models\PricePrediction;
-use Illuminate\Database\Eloquent\Factories\Factory;
-
-/** @extends Factory<PricePrediction> */
-class PricePredictionFactory extends Factory
-{
-    public function definition(): array
-    {
-        return [
-            'predicted_for' => fake()->dateTimeBetween('-30 days')->format('Y-m-d'),
-            'source' => fake()->randomElement(PredictionSource::cases()),
-            'direction' => fake()->randomElement(TrendDirection::cases()),
-            'confidence' => fake()->numberBetween(40, 85),
-            'reasoning' => fake()->sentence(12),
-            'generated_at' => now(),
-        ];
-    }
-
-    public function llm(): static
-    {
-        return $this->state(['source' => PredictionSource::Llm]);
-    }
-
-    public function ewma(): static
-    {
-        return $this->state(['source' => PredictionSource::Ewma]);
-    }
-}