Fix Docker deployment: Update settings.py defaults and Dockerfile for proper environment variable handling

- Updated settings.py to use Docker service names as defaults (postgres_platform, postgres_mtgdata, postgres_mirror, redis)
- Added MTG_MIRROR_DATABASE_URL setting
- Dockerfile copies .env file into container for pydantic-settings
- Entry point script handles dependency checking and migrations before app startup

backend/Dockerfile

@@ -54,7 +54,8 @@ COPY --chown=appuser:appuser alembic/ ./alembic/
 COPY --chown=appuser:appuser scripts/ ./scripts/
 RUN chmod +x /app/scripts/*.py /app/scripts/docker-entrypoint.sh
 
-# Copy environment example
+# Copy environment files
+COPY --chown=appuser:appuser .env ./.env
 COPY --chown=appuser:appuser .env.example ./.env.example
 COPY --chown=appuser:appuser pyproject.toml ./