fix: ensure data persistence across reinstallations

- Add entrypoint.sh: creates /app/data, stamps legacy DBs, runs alembic upgrade head
- Update Dockerfile to use new entrypoint and pre-create /app/data
- Change default DATABASE_URL to sqlite:///./data/dmarq.db
- Add _ensure_sqlite_dir() to database.py for automatic directory creation
- Update docker-compose.yml with app_data named volume for /app/data
- Add 6 tests for _ensure_sqlite_dir and updated default URL

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/2bac317b-97a2-450b-84f1-3e316f7ef54c

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/core/config.py

@@ -23,7 +23,9 @@ class Settings(BaseSettings):
     API_V1_STR: str = "/api/v1"
 
     # Database
-    DATABASE_URL: str = "sqlite:///./dmarq.db"
+    # Default to a sub-directory so the SQLite file lives in a location that
+    # can be persisted via a Docker volume mount (e.g. /app/data).
+    DATABASE_URL: str = "sqlite:///./data/dmarq.db"
 
     # JWT Authentication
     SECRET_KEY: Optional[str] = None