feat: automatic DB backup before migrations using VERSION file

- Add VERSION file at repo root as single source of truth for app version
- frontend/vite.config.ts reads VERSION file instead of package.json
- backend config.py exposes APP_VERSION read from VERSION (dev) or /app/VERSION (Docker)
- database.py backs up DB to homelab.db.back-{version} before running migrations
  (skipped if DB doesn't exist or backup already exists — fully idempotent)
- Dockerfile.backend and Dockerfile.frontend copy VERSION into the image
- Add test_db_backup.py with 4 tests covering create/skip/idempotent/version cases

frontend/vite.config.ts

@@ -1,12 +1,14 @@
+import fs from 'fs'
 import path from 'path'
 import { defineConfig } from 'vitest/config'
 import react from '@vitejs/plugin-react'
 import tailwindcss from '@tailwindcss/vite'
-import pkg from './package.json'
+
+const appVersion = fs.readFileSync(path.resolve(__dirname, '../VERSION'), 'utf-8').trim()
 
 export default defineConfig({
   define: {
-    __APP_VERSION__: JSON.stringify(pkg.version),
+    __APP_VERSION__: JSON.stringify(appVersion),
   },
   plugins: [react(), tailwindcss()],
   resolve: {