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/RegionalMomentumSignal.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace App\Services\Prediction\Signals;
+
+use App\Services\HaversineQuery;
+use Illuminate\Support\Facades\DB;
+
+final class RegionalMomentumSignal extends AbstractSignal
+{
+    private const float SLOPE_THRESHOLD_PENCE = 0.3;
+
+    private const float REGIONAL_RADIUS_KM = 50.0;
+
+    public function compute(SignalContext $context): array
+    {
+        if (! $context->hasCoordinates()) {
+            return $this->disabledSignal('No coordinates provided for regional momentum analysis');
+        }
+
+        [$radiusSql, $radiusBindings] = HaversineQuery::withinKm($context->lat, $context->lng, self::REGIONAL_RADIUS_KM);
+
+        $rows = DB::table('station_prices')
+            ->join('stations', 'station_prices.station_id', '=', 'stations.node_id')
+            ->where('station_prices.fuel_type', $context->fuelType->value)
+            ->where('station_prices.price_effective_at', '>=', now()->subDays(14))
+            ->whereRaw($radiusSql, $radiusBindings)
+            ->selectRaw('DATE(station_prices.price_effective_at) as day, AVG(station_prices.price_pence) as avg_price')
+            ->groupBy('day')
+            ->orderBy('day')
+            ->get();
+
+        if ($rows->count() < 3) {
+            return $this->disabledSignal('Insufficient regional data');
+        }
+
+        $regression = $this->linearRegression($rows->pluck('avg_price')->map(fn ($v) => (float) $v / 100)->values()->all());
+        $direction = match (true) {
+            $regression['slope'] >= self::SLOPE_THRESHOLD_PENCE => 'up',
+            $regression['slope'] <= -self::SLOPE_THRESHOLD_PENCE => 'down',
+            default => 'stable',
+        };
+
+        return [
+            'score' => $direction === 'stable' ? 0.0 : ($direction === 'up' ? 0.7 : -0.7),
+            'confidence' => min(1.0, $regression['r_squared']),
+            'direction' => $direction,
+            'detail' => 'Regional trend: '.round($regression['slope'], 2).'p/day (R²='.round($regression['r_squared'], 2).')',
+            'data_points' => $rows->count(),
+            'enabled' => true,
+        ];
+    }
+}