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/app/core/config.py

@@ -7,6 +7,17 @@ from pydantic_settings import BaseSettings, SettingsConfigDict
 
 logger = logging.getLogger(__name__)
 
+def _read_version() -> str:
+    for candidate in [
+        Path(__file__).parent.parent.parent.parent / "VERSION",  # repo root (dev)
+        Path("/app/VERSION"),                                      # Docker image
+    ]:
+        if candidate.exists():
+            return candidate.read_text().strip()
+    return "unknown"
+
+APP_VERSION = _read_version()
+
 
 class Settings(BaseSettings):
     model_config = SettingsConfigDict(env_file=".env", env_file_encoding="utf-8")

backend/app/db/database.py

@@ -1,3 +1,5 @@
+import logging
+import shutil
 from collections.abc import AsyncGenerator
 from contextlib import suppress
 from pathlib import Path
@@ -6,7 +8,9 @@ from sqlalchemy.exc import OperationalError
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 from sqlalchemy.orm import DeclarativeBase
 
-from app.core.config import settings
+from app.core.config import APP_VERSION, settings
+
+logger = logging.getLogger(__name__)
 
 # Ensure the data directory exists before SQLite tries to open the file
 Path(settings.sqlite_path).parent.mkdir(parents=True, exist_ok=True)
@@ -23,7 +27,22 @@ class Base(DeclarativeBase):
     pass
 
 
+def _backup_db() -> None:
+    db_path = Path(settings.sqlite_path)
+    if not db_path.exists():
+        return
+    backup_path = db_path.with_suffix(f".db.back-{APP_VERSION}")
+    if backup_path.exists():
+        return
+    try:
+        shutil.copy2(db_path, backup_path)
+        logger.info("DB backup created: %s", backup_path.name)
+    except OSError:
+        logger.warning("Could not create DB backup at %s", backup_path)
+
+
 async def init_db() -> None:
+    _backup_db()
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
         # Add columns introduced after initial schema (idempotent)