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/DayOfWeekSignal.php

@@ -0,0 +1,83 @@
+<?php
+
+namespace App\Services\Prediction\Signals;
+
+use Illuminate\Support\Facades\DB;
+
+final class DayOfWeekSignal extends AbstractSignal
+{
+    private const int MIN_DAYS = 21;
+
+    public function compute(SignalContext $context): array
+    {
+        $isSqlite = DB::connection()->getDriverName() === 'sqlite';
+        $dowExpr = $isSqlite
+            ? "(CAST(strftime('%w', price_effective_at) AS INTEGER) + 1)"
+            : 'DAYOFWEEK(price_effective_at)';
+
+        $rows = DB::table('station_prices')
+            ->where('fuel_type', $context->fuelType->value)
+            ->where('price_effective_at', '>=', now()->subDays(90))
+            ->selectRaw("{$dowExpr} as dow, DATE(price_effective_at) as day, AVG(price_pence) as avg_price")
+            ->groupBy('dow', 'day')
+            ->get();
+
+        $uniqueDays = $rows->pluck('day')->unique()->count();
+
+        if ($uniqueDays < self::MIN_DAYS) {
+            return $this->disabledSignal("Insufficient history for day-of-week pattern ({$uniqueDays} days, need ".self::MIN_DAYS.')');
+        }
+
+        $dowAverages = $rows->groupBy('dow')->map(fn ($g) => $g->avg('avg_price'));
+        $weekAvg = $dowAverages->avg();
+        $todayDow = (int) now()->format('w') + 1; // PHP 0=Sun → MySQL 1=Sun
+        $todayAvg = $dowAverages->get($todayDow, $weekAvg);
+        $cheapestDow = $dowAverages->keys()->sortBy(fn ($k) => $dowAverages[$k])->first();
+        $dayNames = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
+        $todayName = $dayNames[($todayDow - 1) % 7] ?? 'Today';
+        $tomorrowName = $dayNames[$todayDow % 7] ?? 'Tomorrow';
+
+        $todayDeltaPence = round(($todayAvg - $weekAvg) / 100, 1);
+        $tomorrowDeltaPence = round(($dowAverages->get(($todayDow % 7) + 1, $weekAvg) - $todayAvg) / 100, 1);
+
+        $direction = match (true) {
+            ($todayAvg - $weekAvg) / 100 >= 1.5 => 'up',
+            ($weekAvg - $todayAvg) / 100 >= 1.5 => 'down',
+            default => 'stable',
+        };
+
+        $score = $direction === 'stable' ? 0.0 : ($direction === 'up' ? 1.0 : -1.0);
+
+        $parts = [];
+        $parts[] = abs($todayDeltaPence) < 0.1
+            ? "Today ({$todayName}) is typically in line with the weekly average."
+            : sprintf(
+                'Today (%s) is typically %sp %s the weekly average.',
+                $todayName,
+                number_format(abs($todayDeltaPence), 1),
+                $todayDeltaPence > 0 ? 'above' : 'below',
+            );
+
+        $parts[] = abs($tomorrowDeltaPence) < 0.1
+            ? "Tomorrow ({$tomorrowName}) is typically the same."
+            : sprintf(
+                'Tomorrow (%s) is typically %sp %s.',
+                $tomorrowName,
+                number_format(abs($tomorrowDeltaPence), 1),
+                $tomorrowDeltaPence < 0 ? 'cheaper' : 'pricier',
+            );
+
+        if ($cheapestDow === $todayDow) {
+            $parts[] = 'Today is historically the cheapest day of the week.';
+        }
+
+        return [
+            'score' => $score,
+            'confidence' => min(1.0, $uniqueDays / 90),
+            'direction' => $direction,
+            'detail' => implode(' ', $parts),
+            'data_points' => $uniqueDays,
+            'enabled' => true,
+        ];
+    }
+}