Fix settings.py defaults for Docker service names

- Updated DATABASE_URL default to postgres_platform:5432
- Updated MTG_DATABASE_URL default to postgres_mtgdata:5432
- Added MTG_MIRROR_DATABASE_URL for postgres_mirror:5432
- Updated REDIS_URL default to redis:6379
- These changes ensure the application works correctly in Docker deployment

Prepares for comprehensive API testing with Docker deployment.

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 (not the actual .env)
+# Copy environment files
+COPY --chown=appuser:appuser .env ./.env
 COPY --chown=appuser:appuser .env.example ./.env.example
 COPY --chown=appuser:appuser pyproject.toml ./