fix: ensure data directory exists before SQLite engine init

On fresh installs the data/ dir may not exist, causing SQLite to create a
stub file without write permissions. mkdir(parents=True, exist_ok=True) runs
at import time before any DB operation.

Also add data/.gitignore to prevent homelab.db from ever being committed.

backend/app/db/database.py

@@ -1,11 +1,15 @@
 from collections.abc import AsyncGenerator
 from contextlib import suppress
+from pathlib import Path
 
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 from sqlalchemy.orm import DeclarativeBase
 
 from app.core.config import settings
 
+# Ensure the data directory exists before SQLite tries to open the file
+Path(settings.sqlite_path).parent.mkdir(parents=True, exist_ok=True)
+
 engine = create_async_engine(
     f"sqlite+aiosqlite:///{settings.sqlite_path}",
     echo=False,

backend/data/.gitignore

@@ -0,0 +1,4 @@
+*.db
+*.db-shm
+*.db-wal
+scan_config.json