Add trains endpoint handler

src/lib/helpers.ts

@@ -0,0 +1,20 @@
+/**
+ * Normalises input to YYYY-MM-DD string
+ * - Strings are passed through as is
+ * - Date objects are converted to UK time
+ */
+export const ensureDateString = (input: Date | string): string => {
+    if (typeof input === 'string') return input.trim();
+
+    const d = new Intl.DateTimeFormat('en-GB', {
+        year: 'numeric',
+        month: '2-digit',
+        day: '2-digit',
+        timeZone: 'Europe/London'
+    }).formatToParts(input);
+
+    const part = (type: string) => d.find(p => p.type === type)?.value;
+
+    return `${part('year')}-${part('month')}-${part('day')}`
+
+}