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

backend/tests/test_db_backup.py

@@ -0,0 +1,57 @@
+"""
+Tests for automatic DB backup before migrations.
+"""
+import os
+
+os.environ.setdefault("SECRET_KEY", "test-only-secret-key-not-for-production")
+
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+from app.db.database import _backup_db
+
+
+@pytest.fixture()
+def tmp_db(tmp_path: Path):
+    db = tmp_path / "homelab.db"
+    db.write_bytes(b"SQLite placeholder")
+    return db
+
+
+def test_backup_created_when_db_exists(tmp_db: Path):
+    with patch("app.db.database.settings") as mock_settings, \
+         patch("app.db.database.APP_VERSION", "1.9"):
+        mock_settings.sqlite_path = str(tmp_db)
+        _backup_db()
+        backup = tmp_db.parent / "homelab.db.back-1.9"
+        assert backup.exists()
+        assert backup.read_bytes() == b"SQLite placeholder"
+
+
+def test_backup_skipped_when_db_missing(tmp_path: Path):
+    with patch("app.db.database.settings") as mock_settings, \
+         patch("app.db.database.APP_VERSION", "1.9"):
+        mock_settings.sqlite_path = str(tmp_path / "nonexistent.db")
+        _backup_db()
+        assert not any(tmp_path.glob("*.back-*"))
+
+
+def test_backup_idempotent_second_call_no_overwrite(tmp_db: Path):
+    with patch("app.db.database.settings") as mock_settings, \
+         patch("app.db.database.APP_VERSION", "1.9"):
+        mock_settings.sqlite_path = str(tmp_db)
+        _backup_db()
+        backup = tmp_db.parent / "homelab.db.back-1.9"
+        backup.write_bytes(b"original backup")
+        _backup_db()
+        assert backup.read_bytes() == b"original backup"
+
+
+def test_backup_version_in_filename(tmp_db: Path):
+    with patch("app.db.database.settings") as mock_settings, \
+         patch("app.db.database.APP_VERSION", "2.0"):
+        mock_settings.sqlite_path = str(tmp_db)
+        _backup_db()
+        assert (tmp_db.parent / "homelab.db.back-2.0").exists()