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/ca.js

@@ -12,14 +12,11 @@ module.exports = function(ctx) {
 
   // Get CA certificate information
   router.get('/info', ctx.asyncHandler(async (req, res) => {
-    const certInfoPath = '/app/ca/cert-info.json';
-    const fallbackCertInfoPath = path.join(platformPaths.caCertDir, 'cert-info.json');
+    const certInfoPath = path.join(platformPaths.caCertDir, 'cert-info.json');
 
     let certInfoFile;
     if (await exists(certInfoPath)) {
       certInfoFile = certInfoPath;
-    } else if (await exists(fallbackCertInfoPath)) {
-      certInfoFile = fallbackCertInfoPath;
     } else {
       const { NotFoundError } = require('../errors');
       throw new NotFoundError('CA certificate information');
@@ -46,13 +43,11 @@ module.exports = function(ctx) {
 
   // Serve root CA certificate directly (works even without DashCA deployed)
   router.get('/root.crt', ctx.asyncHandler(async (req, res) => {
-    const pkiCertPath = '/app/pki/root.crt';
     const hostCertPath = platformPaths.pkiRootCert;
     const dashcaCertPath = path.join(platformPaths.caCertDir, 'root.crt');
 
     let certPath;
-    if (await exists(pkiCertPath)) certPath = pkiCertPath;
-    else if (await exists(dashcaCertPath)) certPath = dashcaCertPath;
+    if (await exists(dashcaCertPath)) certPath = dashcaCertPath;
     else if (await exists(hostCertPath)) certPath = hostCertPath;
     else {
       const { NotFoundError } = require('../errors');
@@ -72,13 +67,12 @@ module.exports = function(ctx) {
     }
 
     // Load cert info to get the fingerprint
-    const certInfoPath = '/app/ca/cert-info.json';
-    const fallbackCertInfoPath2 = path.join(platformPaths.caCertDir, 'cert-info.json');
+    const certInfoPath = path.join(platformPaths.caCertDir, 'cert-info.json');
 
     let certInfoFile;
-    if (await exists(certInfoPath)) certInfoFile = certInfoPath;
-    else if (await exists(fallbackCertInfoPath2)) certInfoFile = fallbackCertInfoPath2;
-    else {
+    if (await exists(certInfoPath)) {
+      certInfoFile = certInfoPath;
+    } else {
       const { NotFoundError } = require('../errors');
       throw new NotFoundError('CA certificate information. Deploy DashCA first or ensure cert-info.json exists.');
     }
@@ -100,7 +94,7 @@ module.exports = function(ctx) {
     // Look for template in multiple locations (packaged app vs dev)
     const templatePaths = [
       path.join(__dirname, '..', 'scripts', templateName),
-      path.join('/app', 'scripts', templateName)
+      path.join(platformPaths.caddyBase, 'scripts', templateName)
     ];
 
     let templateContent;
@@ -142,8 +136,8 @@ module.exports = function(ctx) {
       return ctx.errorResponse(res, 400, `Invalid domain name. Must be a valid hostname (e.g., dns1${ctx.siteConfig.tld})`);
     }
 
-    const pkiPath = '/app/pki';
-    const certsDir = '/app/generated-certs';
+    const pkiPath = platformPaths.pkiDir;
+    const certsDir = platformPaths.generatedCertsDir;
     const domainDir = path.join(certsDir, domain);
 
     const intermediateCert = path.join(pkiPath, 'intermediate.crt');
@@ -246,7 +240,7 @@ ${safeDomain.includes('.') ? `DNS.2 = *.${safeDomain}` : ''}`;
 
   // List generated certificates
   router.get('/certs', ctx.asyncHandler(async (req, res) => {
-    const certsDir = '/app/generated-certs';
+    const certsDir = platformPaths.generatedCertsDir;
 
     if (!await exists(certsDir)) {
       return res.json({ success: true, certificates: [] });