Standardize paths, add version endpoint, request timeouts, HOST env var, graceful shutdown

Cross-platform hardening — removes all hardcoded /app/ paths from route files
and routes them through platform-paths.js so the app works the same way
regardless of Docker layout (single-file mount vs consolidated data dir).

Changes:
- platform-paths.js: add generatedCertsDir, pkiDir, containerUpdatesDir,
  containerFrontendDir, containerAssetsDir, resolveAssetsPath()
- self-updater.js: UPDATE_URL/MIRROR_URL/CHANNEL env var overrides
- routes/ca.js: use platformPaths for cert paths and generated certs dir
- routes/services.js: use platformPaths.pkiRootCert
- routes/themes.js: derive THEMES_DIR from platformPaths.servicesFile
- routes/config/assets.js + backup.js: use resolveAssetsPath() fallback
- routes/services.js + src/app.js: use platformPaths.pkiRootCert
- server.js: HOST env var support, parse PORT as int
- src/app.js: GET /api/v1/version (public, no auth), global request timeout,
  disable x-powered-by, trust proxy
- pylon/dashcaddy-pylon.js: PYLON_HOST env var, graceful shutdown on SIGTERM/SIGINT

A fresh user can now deploy with a custom Docker layout (e.g. /opt/dc/data/
as a single volume mount) and the app finds its files automatically, no env
var configuration required.

dashcaddy-api/pylon/dashcaddy-pylon.js

@@ -226,7 +226,23 @@ const server = http.createServer(async (req, res) => {
   json(res, 404, { error: 'Not found' });
 });
 
-server.listen(PORT, '0.0.0.0', () => {
-  console.log(`[Pylon] ${PYLON_NAME} listening on port ${PORT}`);
+const PYLON_PORT = parseInt(process.env.PYLON_PORT, 10) || 7842;
+const PYLON_HOST = process.env.PYLON_HOST || '0.0.0.0';
+
+server.listen(PYLON_PORT, PYLON_HOST, () => {
+  console.log(`[Pylon] ${PYLON_NAME} listening on ${PYLON_HOST}:${PYLON_PORT}`);
   if (API_KEY) console.log('[Pylon] API key authentication enabled');
 });
+
+// Graceful shutdown — drain connections, then exit
+const shutdown = (signal) => {
+  console.log(`[Pylon] ${signal} received, draining...`);
+  server.close(() => {
+    console.log('[Pylon] HTTP server closed');
+    process.exit(0);
+  });
+  // Force exit after 5s if connections don't drain
+  setTimeout(() => process.exit(0), 5000).unref();
+};
+process.on('SIGTERM', () => shutdown('SIGTERM'));
+process.on('SIGINT', () => shutdown('SIGINT'));