refactor: extract HaversineQuery helper, fix LL bind quirk

The 5 haversine SQL fragments duplicated across StationController and
NationalFuelPredictionService disagreed on float-clamping (LEAST only,
GREATEST/LEAST, vs. CASE WHEN). Centralised in App\Services\HaversineQuery
with the safe GREATEST(-1.0, LEAST(1.0, …)) form everywhere.

withinKm() embeds the radius as a numeric literal (sprintf %F) because
PDO + SQLite binds float parameters as strings by default, which breaks
numeric comparison against the haversine expression — a NULL filter would
silently match all rows. Coordinates remain bound positionally.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/Services/HaversineQuery.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace App\Services;
+
+/**
+ * Builds canonical haversine SQL fragments for distance and within-radius
+ * filtering. Centralises the float-clamping (GREATEST/LEAST) and the column
+ * naming convention used across prediction and station search queries.
+ *
+ * Assumes the joined/queried table exposes columns `lat` and `lng`.
+ */
+final class HaversineQuery
+{
+    private const string DISTANCE_KM_SQL =
+        '(6371 * acos(GREATEST(-1.0, LEAST(1.0, '
+        .'cos(radians(?)) * cos(radians(lat)) * cos(radians(lng) - radians(?)) '
+        .'+ sin(radians(?)) * sin(radians(lat))))))';
+
+    /**
+     * Bare distance-in-km expression. Caller adds aliasing or comparison.
+     *
+     * @return array{0: string, 1: array{float, float, float}}
+     */
+    public static function distanceKm(float $lat, float $lng): array
+    {
+        return [self::DISTANCE_KM_SQL, [$lat, $lng, $lat]];
+    }
+
+    /**
+     * `<= {km}` predicate suitable for whereRaw. The radius is embedded as a
+     * numeric literal because PDO + SQLite's whereRaw binds floats as strings
+     * by default, which breaks numeric comparison against the haversine
+     * expression. The `float` parameter is type-checked and not user input.
+     *
+     * @return array{0: string, 1: array{float, float, float}}
+     */
+    public static function withinKm(float $lat, float $lng, float $km): array
+    {
+        return [self::DISTANCE_KM_SQL.' <= '.sprintf('%F', $km), [$lat, $lng, $lat]];
+    }
+}