**Improve database handling and disable redundant health checks**

- Added logic to construct `DATABASE_URL` from `DB_*` variables to prevent accidental fallback to SQLite during Docker Compose deployments.
- Updated `DATABASES` configuration to use `dj_database_url.parse` instead of deprecated `config`.
- Disabled health checks for services that do not expose a web port to avoid unnecessary probes.
- Removed unused `libmariadb-dev` and `pkg-config` dependencies from the `Dockerfile`.

docker-compose.yml

@@ -30,6 +30,9 @@ services:
       - ${COMIC_BOOK_HOST_PATH}:${COMIC_BOOK_VOLUME}
       - media_files:/media
       - .env:/src/.env
+    # The image HEALTHCHECK probes the web port, which this service never serves.
+    healthcheck:
+      disable: true
     command: sh entrypoint-cron.sh
     restart: unless-stopped