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

Dockerfile.backend

@@ -9,6 +9,7 @@ COPY backend/requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 
 COPY backend/ .
+COPY VERSION /app/VERSION
 
 # Create data directory (volume mount point)
 RUN mkdir -p /app/data