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>

docker-compose.yml

@@ -26,7 +26,8 @@ services:
       context: ./backend
       dockerfile: Dockerfile
     volumes:
-      - ./backend:/app
+      - ./backend:/app          # development: mount source code for live reload
+      - app_data:/app/data      # persist SQLite database and other application data
     depends_on:
       db:
         condition: service_healthy
@@ -50,4 +51,6 @@ networks:
 # Define volumes
 volumes:
   postgres_data:
+    driver: local
+  app_data:
     driver: local