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/routes/config/backup.js

@@ -4,6 +4,7 @@ const path = require('path');
 const { CADDY } = require('../../constants');
 const { exists } = require('../../fs-helpers');
 const { ValidationError, AuthenticationError } = require('../../errors');
+const platformPaths = require('../../platform-paths');
 
 /**
  * Config backup routes factory
@@ -115,7 +116,7 @@ module.exports = function(deps) {
 
     // Include custom assets (logo, favicon) as base64
     try {
-      const assetsDir = process.env.ASSETS_DIR || '/app/assets';
+      const assetsDir = platformPaths.resolveAssetsPath(process.env.ASSETS_DIR);
       const configData = backup.files.config?.data || {};
       const assetFiles = [configData.customLogo, configData.customFavicon]
         .filter(Boolean)
@@ -346,7 +347,7 @@ module.exports = function(deps) {
 
     // Restore custom assets from base64
     if (backup.assets && typeof backup.assets === 'object') {
-      const assetsDir = process.env.ASSETS_DIR || '/app/assets';
+      const assetsDir = platformPaths.resolveAssetsPath(process.env.ASSETS_DIR);
       for (const [name, b64] of Object.entries(backup.assets)) {
         try {
           const safeName = path.basename(name); // prevent path traversal