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/BrandBehaviourSignal.php

@@ -0,0 +1,61 @@
+<?php
+
+namespace App\Services\Prediction\Signals;
+
+use Illuminate\Support\Facades\DB;
+
+final class BrandBehaviourSignal extends AbstractSignal
+{
+    public function compute(SignalContext $context): array
+    {
+        $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(7))
+            ->selectRaw('stations.is_supermarket, DATE(station_prices.price_effective_at) as day, AVG(station_prices.price_pence) as avg_price')
+            ->groupBy('stations.is_supermarket', 'day')
+            ->orderBy('day')
+            ->get();
+
+        $supermarket = $rows->where('is_supermarket', 1)->values();
+        $major = $rows->where('is_supermarket', 0)->values();
+
+        if ($supermarket->count() < 2 || $major->count() < 2) {
+            return $this->disabledSignal('Insufficient brand data for comparison');
+        }
+
+        $supermarketSlope = $this->linearRegression($supermarket->pluck('avg_price')->map(fn ($v) => (float) $v / 100)->values()->all())['slope'];
+        $majorSlope = $this->linearRegression($major->pluck('avg_price')->map(fn ($v) => (float) $v / 100)->values()->all())['slope'];
+
+        $divergence = round(abs($supermarketSlope - $majorSlope) * 7, 1);
+        $supermarketChange = round($supermarketSlope * 7, 1);
+        $majorChange = round($majorSlope * 7, 1);
+
+        if ($divergence < 1.0) {
+            return [
+                'score' => 0.0,
+                'confidence' => 0.5,
+                'direction' => 'stable',
+                'detail' => 'Supermarkets and majors moving in sync.',
+                'data_points' => $rows->count(),
+                'enabled' => true,
+            ];
+        }
+
+        $leaderChange = abs($supermarketChange) > abs($majorChange) ? $supermarketChange : $majorChange;
+        $direction = $leaderChange > 0 ? 'up' : 'down';
+        $leader = abs($supermarketChange) > abs($majorChange) ? 'Supermarkets' : 'Majors';
+        $follower = $leader === 'Supermarkets' ? 'majors' : 'supermarkets';
+        $leaderAbs = abs($leaderChange);
+        $followerChange = $leader === 'Supermarkets' ? abs($majorChange) : abs($supermarketChange);
+
+        return [
+            'score' => $direction === 'up' ? 1.0 : -1.0,
+            'confidence' => min(1.0, $divergence / 5.0),
+            'direction' => $direction,
+            'detail' => "{$leader} ".($leaderChange > 0 ? 'rose' : 'fell')." {$leaderAbs}p vs {$follower} {$followerChange}p (divergence: {$divergence}p). Expect {$follower} to follow.",
+            'data_points' => $rows->count(),
+            'enabled' => true,
+        ];
+    }
+}