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/Signal.php

@@ -1,24 +0,0 @@
-<?php
-
-namespace App\Services\Prediction\Signals;
-
-interface Signal
-{
-    /**
-     * Evaluate the signal against the given context.
-     *
-     * Returns the canonical signal payload. Implementations may add extra
-     * keys beyond the base shape (e.g. trend adds slope + r_squared).
-     *
-     * @return array{
-     *     score: float,
-     *     confidence: float,
-     *     direction: string,
-     *     detail: string,
-     *     data_points: int,
-     *     enabled: bool,
-     *     ...
-     * }
-     */
-    public function compute(SignalContext $context): array;
-}