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/migrations/2026_05_03_071205_drop_price_predictions_table.php

@@ -0,0 +1,30 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Retire the legacy oil-prediction pipeline. The new forecasting
+     * stack writes to `weekly_forecasts` (Layer 1 ridge) and
+     * `llm_overlays` (Layer 4 LLM news scan) instead. Per
+     * `docs/superpowers/specs/2026-05-01-prediction-rebuild-design.md`
+     * this table and its associated services were due for removal at
+     * the end of Phase 4.
+     */
+    public function up(): void
+    {
+        Schema::dropIfExists('price_predictions');
+    }
+
+    /**
+     * Re-creating the dropped table is not supported — the legacy
+     * services that populated it have been deleted along with the
+     * table. To restore, revert the prediction-rebuild commits.
+     */
+    public function down(): void
+    {
+        // intentionally empty; see docblock above
+    }
+};