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/themes.js

@@ -3,6 +3,7 @@ const fs = require('fs');
 const path = require('path');
 const { success } = require('../response-helpers');
 const { ValidationError, NotFoundError } = require('../errors');
+const platformPaths = require('../platform-paths');
 
 /**
  * Themes routes factory
@@ -13,7 +14,7 @@ const { ValidationError, NotFoundError } = require('../errors');
  */
 module.exports = function({ asyncHandler, log }) {
   const router = express.Router();
-  const THEMES_DIR = process.env.THEMES_DIR || path.join(path.dirname(process.env.SERVICES_FILE || '/app/services.json'), 'themes');
+  const THEMES_DIR = process.env.THEMES_DIR || path.join(path.dirname(platformPaths.servicesFile), 'themes');
 
   // Ensure themes directory exists
   if (!fs.existsSync(THEMES_DIR)) {