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/SignalContext.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Services\Prediction\Signals;
+
+use App\Enums\FuelType;
+
+/**
+ * Inputs required to evaluate a prediction signal. Individual signals may
+ * ignore fields they don't need — for example OilSignal doesn't use fuelType,
+ * RegionalMomentumSignal requires lat/lng to be non-null.
+ */
+final readonly class SignalContext
+{
+    public function __construct(
+        public FuelType $fuelType,
+        public ?float $lat = null,
+        public ?float $lng = null,
+    ) {}
+
+    public function hasCoordinates(): bool
+    {
+        return $this->lat !== null && $this->lng !== null;
+    }
+}