feat: add GET /api/prediction endpoint

Implements PredictionRequest (fuel_type validation with ValueError→ValidationException), PredictionController delegating to NationalFuelPredictionService, and 5 feature tests. Also fixes LEAST() MySQL-only function to a CASE WHEN expression for SQLite test compatibility.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/Feature/Api/PredictionControllerTest.php

@@ -0,0 +1,59 @@
+<?php
+
+use App\Enums\FuelType;
+use App\Models\Station;
+use App\Models\StationPriceCurrent;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+
+uses(RefreshDatabase::class);
+
+it('returns a prediction response for diesel', function () {
+    $this->getJson('/api/prediction?fuel_type=diesel')
+        ->assertOk()
+        ->assertJsonStructure([
+            'fuel_type', 'current_avg', 'predicted_direction', 'predicted_change_pence',
+            'confidence_score', 'confidence_label', 'action', 'reasoning',
+            'prediction_horizon_days', 'region_key', 'methodology',
+            'signals' => [
+                'trend' => ['score', 'confidence', 'direction', 'detail', 'data_points', 'enabled'],
+                'day_of_week' => ['score', 'confidence', 'direction', 'detail', 'data_points', 'enabled'],
+                'brand_behaviour' => ['score', 'confidence', 'direction', 'detail', 'data_points', 'enabled'],
+                'national_momentum' => ['score', 'confidence', 'direction', 'detail', 'data_points', 'enabled'],
+                'regional_momentum' => ['score', 'confidence', 'direction', 'detail', 'data_points', 'enabled'],
+                'price_stickiness' => ['score', 'confidence', 'direction', 'detail', 'data_points', 'enabled'],
+            ],
+        ])
+        ->assertJsonPath('fuel_type', 'b7_standard')
+        ->assertJsonPath('region_key', 'national');
+});
+
+it('includes current average from live prices', function () {
+    $station = Station::factory()->create();
+    StationPriceCurrent::factory()->create([
+        'station_id' => $station->node_id,
+        'fuel_type' => FuelType::B7Standard,
+        'price_pence' => 14750,
+    ]);
+
+    $response = $this->getJson('/api/prediction?fuel_type=diesel')->assertOk();
+
+    expect($response->json('current_avg'))->toBe(147.5);
+});
+
+it('accepts optional lat and lng for regional context', function () {
+    $this->getJson('/api/prediction?fuel_type=diesel&lat=52.5&lng=-0.2')
+        ->assertOk()
+        ->assertJsonPath('region_key', 'national'); // still national, regional_momentum signal updated internally
+});
+
+it('returns 422 when fuel_type is missing', function () {
+    $this->getJson('/api/prediction')
+        ->assertUnprocessable()
+        ->assertJsonValidationErrors(['fuel_type']);
+});
+
+it('returns 422 for unknown fuel_type alias', function () {
+    $this->getJson('/api/prediction?fuel_type=rocket_fuel')
+        ->assertUnprocessable()
+        ->assertJsonValidationErrors(['fuel_type']);
+});