Refactor data loading: move from inline JSON to client-side fetch

- Remove large JSON data injection from EJS templates
- Add client-side data-loader utility with cache-busting support
- Create server endpoints for JSON data with long-term caching
- Add graceful failure handling if core data fails to load
- Copy JSON data files to dist/data for static hosting
- Update app initialization to load data asynchronously
- Set serverAvailable flag for static builds in gulp task

This reduces HTML payload size and enables better caching strategies
for both server and static deployment modes.

index.mjs

@@ -58,9 +58,6 @@ const renderIndex = (req, res, production = false) => {
 		version,
 		OVERRIDES,
 		query: req.query,
-		travelCities,
-		regionalCities,
-		stationInfo,
 	});
 };
 
@@ -137,6 +134,23 @@ if (!process.env?.STATIC) {
 	app.get('/playlist.json', playlist);
 }
 
+// Data endpoints - serve JSON data with long-term caching
+const dataEndpoints = {
+	travelcities: travelCities,
+	regionalcities: regionalCities,
+	stations: stationInfo,
+};
+
+Object.entries(dataEndpoints).forEach(([name, data]) => {
+	app.get(`/data/${name}.json`, (req, res) => {
+		res.set({
+			'Cache-Control': 'public, max-age=31536000, immutable',
+			'Content-Type': 'application/json',
+		});
+		res.json(data);
+	});
+});
+
 if (process.env?.DIST === '1') {
 	// Production ("distribution") mode uses pre-baked files in the dist directory
 	// 'npm run build' and then 'DIST=1 npm start'