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/Dockerfile

@@ -19,7 +19,11 @@ RUN pip install --no-cache-dir -r requirements.txt
 # Copy application code including templates and static assets
 COPY . .
 
+# Make the entrypoint executable and create the default data directory so
+# SQLite has a place to write its file even without an explicit volume mount.
+RUN chmod +x /app/entrypoint.sh && mkdir -p /app/data
+
 # Expose application port
 EXPOSE 8080
 
-CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8080"]
+ENTRYPOINT ["/app/entrypoint.sh"]