feat: dynamic app version sourced from root VERSION file

Replaces the hardcoded version string in SettingsPage.tsx with a single
source of truth: a /VERSION file at the repo root. To cut a new release,
edit only that one file.

Frontend:
- vite.config.ts reads /VERSION at dev-server startup and injects it as
  the __APP_VERSION__ global via Vite's define mechanism (compile-time,
  zero runtime cost). Falls back to VITE_APP_VERSION env var for prod
  Docker builds that pass it as a build arg.
- vite-env.d.ts adds the TypeScript declaration for __APP_VERSION__.
- SettingsPage.tsx Badge now renders {__APP_VERSION__} instead of the
  literal string.

Backend:
- app.controller.ts reads /VERSION once at module load and includes
  "version" in both GET /api and GET /api/health responses.
- NewRelicTransactionInterceptor tags every NR transaction with
  newrelic.addCustomAttribute('appVersion', version) so releases can be
  compared in NRQL: SELECT average(duration) FROM Transaction WHERE
  appVersion = '2026.5.22'

Docker:
- docker-compose.yml mounts ./VERSION:/app/VERSION:ro in both backend
  and frontend dev containers.
- Production Dockerfiles include COPY VERSION ./ with a comment
  instructing CI to copy the root VERSION into each build context before
  docker build.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -1,9 +1,27 @@
 import { defineConfig } from 'vite';
 import react from '@vitejs/plugin-react';
 import path from 'path';
+import fs from 'fs';
+
+// Read the canonical version from /VERSION (repo root, mounted at /app/VERSION in Docker).
+// Falls back to the VITE_APP_VERSION env var so production Docker builds can pass it
+// as a build arg (--build-arg VITE_APP_VERSION=$(cat VERSION)) without needing the file.
+function readAppVersion(): string {
+  try {
+    return fs.readFileSync(path.resolve(__dirname, 'VERSION'), 'utf-8').trim();
+  } catch {
+    return process.env.VITE_APP_VERSION ?? 'dev';
+  }
+}
+
+const APP_VERSION = readAppVersion();
 
 export default defineConfig({
   plugins: [react()],
+  define: {
+    // Injected at compile time — use __APP_VERSION__ anywhere in frontend source.
+    __APP_VERSION__: JSON.stringify(APP_VERSION),
+  },
   resolve: {
     alias: {
       '@': path.resolve(__dirname, './src'),