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/server.js

@@ -25,7 +25,8 @@ process.on('uncaughtException', (error) => {
     // Load license
     await licenseManager.load();
 
-    const PORT = process.env.PORT || 3001;
+    const PORT = parseInt(process.env.PORT, 10) || 3001;
+    const HOST = process.env.HOST || '0.0.0.0';
     const CADDYFILE_PATH = process.env.CADDYFILE_PATH || platformPaths.caddyfile;
     const CADDY_ADMIN_URL = process.env.CADDY_ADMIN_URL || platformPaths.caddyAdminUrl;
     const SERVICES_FILE = process.env.SERVICES_FILE || platformPaths.servicesFile;
@@ -43,9 +44,10 @@ process.on('uncaughtException', (error) => {
     });
 
     // Start HTTP server
-    const server = app.listen(PORT, '0.0.0.0', () => {
+    const server = app.listen(PORT, HOST, () => {
       log.info('server', 'DashCaddy API server started', {
         port: PORT,
+        host: HOST,
         caddyfile: CADDYFILE_PATH,
         caddyAdmin: CADDY_ADMIN_URL,
         services: SERVICES_FILE,