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/Models/PricePrediction.php

@@ -1,54 +0,0 @@
-<?php
-
-namespace App\Models;
-
-use App\Enums\PredictionSource;
-use App\Enums\TrendDirection;
-use Database\Factories\PricePredictionFactory;
-use Illuminate\Database\Eloquent\Attributes\Fillable;
-use Illuminate\Database\Eloquent\Builder;
-use Illuminate\Database\Eloquent\Factories\HasFactory;
-use Illuminate\Database\Eloquent\Model;
-
-#[Fillable(['predicted_for', 'source', 'direction', 'confidence', 'reasoning', 'generated_at'])]
-class PricePrediction extends Model
-{
-    /** @use HasFactory<PricePredictionFactory> */
-    use HasFactory;
-
-    public $timestamps = false;
-
-    protected function casts(): array
-    {
-        return [
-            'predicted_for' => 'date',
-            'source' => PredictionSource::class,
-            'direction' => TrendDirection::class,
-            'confidence' => 'integer',
-            'generated_at' => 'datetime',
-        ];
-    }
-
-    /**
-     * Order by source quality: llm_with_context → llm → ewma.
-     * Use this whenever reading the "best" prediction for a given date.
-     *
-     * @param  Builder<PricePrediction>  $query
-     * @return Builder<PricePrediction>
-     */
-    public function scopeBestFirst(Builder $query): Builder
-    {
-        $priority = [
-            PredictionSource::LlmWithContext->value,
-            PredictionSource::Llm->value,
-            PredictionSource::Ewma->value,
-        ];
-
-        $cases = '';
-        foreach ($priority as $rank => $source) {
-            $cases .= " WHEN '$source' THEN $rank";
-        }
-
-        return $query->orderByRaw("CASE source$cases ELSE ".count($priority).' END');
-    }
-}