feat: add PostcodeService and price validation with DB constraints

- Add PostcodeService to resolve UK postcodes, outcodes, and place names to coordinates via postcodes.io API with 30-day caching
- Add LocationResult value object for resolved location data
- Add per-fuel-type price validation (80p-1050p range) to FuelPriceService with warning logs for out-of-range prices
- Change price_pence column from unsignedSmallInteger to unsignedMediumInteger in station_prices tables
- Add CHECK constraints (5000-50000 range) on price_pence columns as database-level guard
- Improve error handling in PollFuelPrices command with file/line/trace output
- Add tests for PostcodeService covering full postcodes, outcodes, place names, caching, and error handling
- Add test for price validation range checks

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

app/Services/LocationResult.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace App\Services;
+
+readonly class LocationResult
+{
+    public function __construct(
+        public string $query,
+        public string $displayName,
+        public float $lat,
+        public float $lng,
+    ) {}
+}