refactor: extract 6 prediction signals into Signal classes

The 803-line NationalFuelPredictionService had six private compute*Signal
methods, a private linearRegression helper, and a private disabledSignal
shape factory all crammed together. Each signal is now an independently
testable class.

- App\Services\Prediction\Signals\Signal — interface
- App\Services\Prediction\Signals\SignalContext — input value object
  (FuelType + optional lat/lng + hasCoordinates() helper)
- App\Services\Prediction\Signals\AbstractSignal — shared
  disabledSignal() and linearRegression() helpers
- TrendSignal, DayOfWeekSignal, BrandBehaviourSignal, StickinessSignal,
  RegionalMomentumSignal, OilSignal — one class each, extending
  AbstractSignal

NationalFuelPredictionService receives the 6 signal classes via constructor
injection and orchestrates them. The lat/lng null-guard for regional
momentum now lives inside RegionalMomentumSignal::compute() so the
coordinator no longer branches on coordinate presence.

Aggregation, weekly summary, and reasoning helpers stay in the service
for now — they are coupled to the public predict() output shape and are
candidates for a follow-up extraction once a stable API is locked in.

Service: 803 → 414 lines.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/Services/Prediction/Signals/OilSignal.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace App\Services\Prediction\Signals;
+
+use Illuminate\Support\Facades\DB;
+
+final class OilSignal extends AbstractSignal
+{
+    /**
+     * Reads the most recent Brent crude prediction (LLM preferred, EWMA
+     * fallback) covering today or later. Sourced from price_predictions,
+     * which OilPriceService populates daily.
+     */
+    public function compute(SignalContext $context): array
+    {
+        $prediction = null;
+
+        foreach (['llm_with_context', 'llm', 'ewma'] as $source) {
+            $prediction = DB::table('price_predictions')
+                ->where('source', $source)
+                ->where('predicted_for', '>=', now()->toDateString())
+                ->orderByDesc('predicted_for')
+                ->orderByDesc('generated_at')
+                ->first();
+
+            if ($prediction !== null) {
+                break;
+            }
+        }
+
+        if ($prediction === null) {
+            return $this->disabledSignal('No oil price prediction available');
+        }
+
+        $direction = match ($prediction->direction) {
+            'rising' => 'up',
+            'falling' => 'down',
+            default => 'stable',
+        };
+
+        $score = match ($direction) {
+            'up' => 1.0,
+            'down' => -1.0,
+            default => 0.0,
+        };
+
+        $confidence = round(((float) $prediction->confidence) / 100, 2);
+
+        return [
+            'score' => $score,
+            'confidence' => $confidence,
+            'direction' => $direction,
+            'detail' => sprintf(
+                'Brent crude %s (%s, %d%% confidence)',
+                $prediction->direction,
+                $prediction->source,
+                (int) $prediction->confidence,
+            ),
+            'data_points' => 1,
+            'enabled' => true,
+        ];
+    }
+}