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/platform-paths.js

@@ -3,6 +3,7 @@
 // All paths can be overridden via environment variables.
 
 const path = require('path');
+const fs = require('fs');
 const isWindows = process.platform === 'win32';
 
 // Base directories
@@ -34,6 +35,8 @@ const paths = {
   caCertDir: path.join(CADDY_SITES, 'ca'),
   pkiRootCert: path.join(CADDY_PKI, 'root.crt'),
   pkiIntermediateCert: path.join(CADDY_PKI, 'intermediate.crt'),
+  generatedCertsDir: path.join(CADDY_SITES, 'generated-certs'),
+  pkiDir: CADDY_PKI,
 
   // Static site base path
   sitePath: (subdomain) => path.join(CADDY_SITES, subdomain),
@@ -41,6 +44,24 @@ const paths = {
   // Docker data path for app volumes
   appData: (appName) => path.join(DOCKER_DATA, appName),
 
+  // In-container paths (used by self-updater and Docker deployments)
+  // Override via env vars for custom Docker layouts
+  containerUpdatesDir: process.env.DASHCADDY_UPDATES_DIR || '/app/updates',
+  containerFrontendDir: process.env.DASHCADDY_FRONTEND_DIR || '/app/dashboard',
+  containerAssetsDir: process.env.ASSETS_DIR || '/app/assets',
+
+  // Asset path resolution — supports both Docker (single file mount) and
+  // consolidated data directory layouts
+  resolveAssetsPath: (envPath) => {
+    if (envPath) return envPath;
+    // Standard Docker mount: /app/assets (volume-mounted)
+    if (fs.existsSync('/app/assets')) return '/app/assets';
+    // Consolidated data directory: /app/data/assets
+    if (fs.existsSync(path.join(CADDY_BASE, 'assets'))) return path.join(CADDY_BASE, 'assets');
+    // Fall back to /app/assets even if it doesn't exist (will create on write)
+    return '/app/assets';
+  },
+
   // Log digest directory
   digestDir: process.env.DIGEST_DIR || path.join(CADDY_BASE, 'digests'),